Pollywog Symbiote

Each creature spell you cast costs {1} less to cast if it has mutate.
Whenever you cast a creature spell, if it has mutate, draw a card, then discard a card.

Smash the Landmark

Target Shark you control explores.

Create a Map token.

Arcane Signet

{T}: Add one mana of any color in your commander's color identity.

Pouncing Shoreshark

Mutate {3}{U} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Flash
Whenever this creature mutates, you may return target creature an opponent controls to its owner's hand.

Forest

Command Tower

Shadow Set Unlocked

Enchant artifact or creature

As long as enchanted permanent is a creature, it gets -1/-1 and can't block.

Whenever enchanted permanent becomes tapped, its controller gets a poison counter.

Killa, Apex Orca

Menace

Whenever Killa, Apex Orca or another creature dies, target player loses 1 life.

When Killa dies, return another target Shark, Leviathan, or Octopus card from your graveyard to your hand.

Mycoid Maze

(Transforms from Twists and Turns.)
{T}: Add {G}.
{3}{G}, {T}: Look at the top four cards of your library. You may reveal a creature card from among them and put that card into your hand. Put the rest on the bottom of your library in a random order.

Twists and Turns

If a creature you control would explore, instead you scry 1, then that creature explores.
When this enchantment enters, target creature you control explores.
When a land you control enters, if you control seven or more lands, transform this enchantment.

Twists and Turns // Mycoid Maze

License Plate Lunch

Target creature gets +2/+0 until end of turn. If that creature is a Shark, it also gains haste until end of turn. 

Create a Mutagen token.

Spectator Seating

This land enters tapped unless you have two or more opponents.
{T}: Add {R} or {W}.

Donatello, the Brains

If one or more tokens would be created under your control, those tokens plus a Mutagen token are created instead.
Partner—Character select (You can have two commanders if both have this ability.)

Auspicious Starrix

Mutate {5}{G} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Whenever this creature mutates, exile cards from the top of your library until you exile X permanent cards, where X is the number of times this creature has mutated. Put those permanent cards onto the battlefield.

Jenny, Generated Anomaly

Double strike
Whenever Jenny deals combat damage to a player, it explores. (Reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then put the card back or put it into your graveyard.)

Fierce Guardianship

If you control a commander, you may cast this spell without paying its mana cost.
Counter target noncreature spell.

Branching Evolution

If one or more +1/+1 counters would be put on a creature you control, twice that many +1/+1 counters are put on that creature instead.

Training Center

This land enters tapped unless you have two or more opponents.
{T}: Add {U} or {R}.

Mutagen Man, Living Ooze

Trample
Activated abilities of artifact tokens you control cost {1} less to activate.
When Mutagen Man enters, create X Mutagen tokens. (They're artifacts with "{1}, {T}, Sacrifice this token: Put a +1/+1 counter on target creature. Activate only as a sorcery.")

Fisher's Talent

(Gain the next level as a sorcery to add its ability.)
At the beginning of your upkeep, look at the top card of your library. You may reveal it if it's a land card. Create a 1/1 blue Fish creature token if you revealed it this way. Then draw a card.
{G}{U}: Level 2
If you would create a Fish token, create a 3/3 blue Shark creature token instead.
{2}{G}{U}: Level 3
If you would create a Shark token, create an 8/8 blue Octopus creature token instead.

Parallel Lives

If an effect would create one or more tokens under your control, it creates twice that many of those tokens instead.

Heroic Intervention

Permanents you control gain hexproof and indestructible until end of turn.

Sol Ring

Terror of Dead Horse Lake

Shark, Leviathan, and Octopus spells you cast cost {1} less to cast.

{T}: Target creature gains haste until end of turn.

Morphic Pool

This land enters tapped unless you have two or more opponents.
{T}: Add {U} or {B}.

Voraskol, Scion of Sekolah

Mutate {2/W}{2/U}{2/B}{2/R}{2/G}
Whenever this creature mutates, draw a card, then discover 5.

Trample, indestructible
If a Shark, Octopus, or Leviathan you control would explore, it explores an additional time.

Maneater, Apex Tiger

Trample, haste
Garbage Disposa — Devour 3

Enrage — Whenever Maneater, Apex Tiger is dealt damage, create a 3/3 blue Shark creature token. If Maneater has seven or more +1/+1 counters on it, create an 8/8 blue Octopus creature token instead.

Gemrazer

Mutate {1}{G}{G} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Reach, trample
Whenever this creature mutates, destroy target artifact or enchantment an opponent controls.

Migratory Greathorn

Mutate {2}{G} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Whenever this creature mutates, search your library for a basic land card, put it onto the battlefield tapped, then shuffle.

Arcane Adaptation

As this enchantment enters, choose a creature type.
Creatures you control are the chosen type in addition to their other types. The same is true for creature spells you control and creature cards you own that aren't on the battlefield.

Nature's Lore

Search your library for a Forest card, put that card onto the battlefield, then shuffle.

Landmark Complete

Each Shark creature you control explores, then for each creature that explored this way, create a Mutagen token.

Path of Discovery

Whenever a creature you control enters, it explores. (Reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on the creature, then put the card back or put it into your graveyard.)

Rapid Hybridization

Destroy target creature. It can't be regenerated. That creature's controller creates a 3/3 green Frog Lizard creature token.

Roaming Throne

Ward {2}
As this creature enters, choose a creature type.
This creature is the chosen type in addition to its other types.
If a triggered ability of another creature you control of the chosen type triggers, it triggers an additional time.

Venomwake, Apex Horror

Mutate {2}{W/B}{G}
Whenever this creature mutates, it gains toxic 2.

Menace, lifelink
Whenever this creature deals combat damage to a player, proliferate.

Hullbreaker Horror

Flash
This spell can't be countered.
Whenever you cast a spell, choose up to one —
• Return target spell you don't control to its owner's hand.
• Return target nonland permanent to its owner's hand.

Terror Level 10

Choose one --
{i}Destroy all sharks creatures
{i}Destroy all non-Shark creatures

Doubling Season

If an effect would create one or more tokens under your control, it creates twice that many of those tokens instead.
If an effect would put one or more counters on a permanent you control, it puts twice that many of those counters on that permanent instead.

Luxury Suite

This land enters tapped unless you have two or more opponents.
{T}: Add {B} or {R}.

City of Brass

Whenever this land becomes tapped, it deals 1 damage to you.
{T}: Add one mana of any color.

Mana Confluence

{T}, Pay 1 life: Add one mana of any color.

Rejuvenating Springs

This land enters tapped unless you have two or more opponents.
{T}: Add {G} or {U}.

Rosie, Apex Gator

Menace
Toxic 1

Whenever Rosie, Apex Gator attacks, mill three cards. Rosie deals damage equal to the total mana value of cards milled this way to any target.

Exotic Orchard

{T}: Add one mana of any color that a land an opponent controls could produce.

Sea of Clouds

This land enters tapped unless you have two or more opponents.
{T}: Add {W} or {U}.

Vorinclex, Monstrous Raider

Trample, haste
If you would put one or more counters on a permanent or player, put twice that many of each of those kinds of counters on that permanent or player instead.
If an opponent would put one or more counters on a permanent or player, they put half that many of each of those kinds of counters on that permanent or player instead, rounded down.

Deathcap Glade

This land enters tapped unless you control two or more other lands.
{T}: Add {B} or {G}.

Spire Garden

This land enters tapped unless you have two or more opponents.
{T}: Add {R} or {G}.

Michelangelo, Weirdness to 11

When Michelangelo enters, create a Mutagen token. (It's an artifact with "{1}, {T}, Sacrifice this token: Put a +1/+1 counter on target creature. Activate only as a sorcery.")
If one or more +1/+1 counters would be put on a creature you control, that many plus one +1/+1 counters are put on it instead.

Tourist Trap

Tap target creature and put a stun counter on it. If you control a Shark, create a Map token.

Bloom Tender

Vivid — {T}: For each color among permanents you control, add one mana of that color.

Teferi's Protection

Until your next turn, your life total can't change and you gain protection from everything. All permanents you control phase out. (While they're phased out, they're treated as though they don't exist. They phase in before you untap during your untap step.)
Exile Teferi's Protection.

Shipwreck Marsh

This land enters tapped unless you control two or more other lands.
{T}: Add {U} or {B}.

Island

Genghis Frog

Trample
Whenever Genghis Frog or another Mutant you control enters, create a Mutagen token. (It's an artifact with "{1}, {T}, Sacrifice this token: Put a +1/+1 counter on target creature. Activate only as a sorcery.")

Weird Thing in the Bayou

Mill three cards. You may return a creature or land card from your graveyard to your hand.

Create a Mutagen token.

Realmwalker

Changeling (This card is every creature type.)
As this creature enters, choose a creature type.
You may look at the top card of your library any time.
You may cast creature spells of the chosen type from the top of your library.

Dreamtail Heron

Mutate {3}{U} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Flying
Whenever this creature mutates, draw a card.

Innkeeper's Talent

(Gain the next level as a sorcery to add its ability.)
At the beginning of combat on your turn, put a +1/+1 counter on target creature you control.
{G}: Level 2
Permanents you control with counters on them have ward {1}.
{3}{G}: Level 3
If you would put one or more counters on a permanent or player, put twice that many of each of those kinds of counters on that permanent or player instead.

Adrix and Nev, Twincasters

Ward {2} (Whenever this creature becomes the target of a spell or ability an opponent controls, counter it unless that player pays {2}.)
If one or more tokens would be created under your control, twice that many of those tokens are created instead.

Hidden Nutrient Cache

Search your library for a basic land card, reveal it, put it in your hand, then shuffle. If you control a Shark, you may put that card on the battlefield tapped instead.

Create a Mutagen token.

Reflecting Pool

{T}: Add one mana of any type that a land you control could produce.

Swords to Plowshares

Exile target creature. Its controller gains life equal to its power.

Enter the Unknown

Target creature you control explores. (Reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on the creature, then put the card back or put it into your graveyard.)
You may play an additional land this turn.

Sea-Dasher Octopus

Mutate {1}{U} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Flash
Whenever this creature deals combat damage to a player, draw a card.

Defossilize

Return target creature card from your graveyard to the battlefield. That creature explores, then it explores again. (Reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on that creature, then put the card back or put it into your graveyard. Then repeat this process.)

Stormcoil, Electropredator

Mutate {1}{U}{B/G}{B/G}
Whenever this creature mutates, tap up to two target creatures an opponent controls and put a stun counter on each of them.
Ward—Put a stun counter on a creature you control.
Deathtouch
Creatures with counters on them can’t block Stormcoil, Electropredator.
At the beginning of your end step, draw a card for each tapped creature your opponents control with a counter on it.

Leonardo, the Balance

Whenever a token you control enters, you may put a +1/+1 counter on each creature you control. Do this only once each turn.
{W}{U}{B}{R}{G}: Creatures you control gain menace, trample, and lifelink until end of turn.
Partner—Character select (You can have two commanders if both have this ability.)

Bountiful Promenade

This land enters tapped unless you have two or more opponents.
{T}: Add {G} or {W}.

Assassin's Trophy

Destroy target permanent an opponent controls. Its controller may search their library for a basic land card, put it onto the battlefield, then shuffle.

Faeburrow Elder

Vigilance
This creature gets +1/+1 for each color among permanents you control.
{T}: For each color among permanents you control, add one mana of that color.

Stormcarved Coast

This land enters tapped unless you control two or more other lands.
{T}: Add {U} or {R}.

Vault of Champions

This land enters tapped unless you have two or more opponents.
{T}: Add {W} or {B}.

Maneater, Bane of Scaly Pete

Mutate {2}{U/G}{R}
Whenever this creature mutates, put a +1/+1 counter on each creature you control.

Haste, ward {1}
Creatures with power less than the number of +1/+1 counters on this creature can’t block creatures you control.

Sundown Pass

This land enters tapped unless you control two or more other lands.
{T}: Add {R} or {W}.

Farseek

Search your library for a Plains, Island, Swamp, or Mountain card, put it onto the battlefield tapped, then shuffle.

Boseiju, Who Endures

{T}: Add {G}.
Channel — {1}{G}, Discard this card: Destroy target artifact, enchantment, or nonbasic land an opponent controls. That player may search their library for a land card with a basic land type, put it onto the battlefield, then shuffle. This ability costs {1} less to activate for each legendary creature you control.

Overgrown Farmland

This land enters tapped unless you control two or more other lands.
{T}: Add {G} or {W}.

Subterranean Schooner

Whenever this Vehicle attacks, target creature that crewed it this turn explores. (Reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on that creature, then put the card back or put it into your graveyard.)
Crew 1

The Ozolith

Whenever a creature you control leaves the battlefield, if it had counters on it, put those counters on The Ozolith.
At the beginning of combat on your turn, if The Ozolith has counters on it, you may move all counters from The Ozolith onto target creature.

Pongify

Destroy target creature. It can't be regenerated. Its controller creates a 3/3 green Ape creature token.

Haunted Ridge

This land enters tapped unless you control two or more other lands.
{T}: Add {B} or {R}.

Swan Song

Counter target enchantment, instant, or sorcery spell. Its controller creates a 2/2 blue Bird creature token with flying.

Undergrowth Stadium

This land enters tapped unless you have two or more opponents.
{T}: Add {B} or {G}.

Apex Great White // Apex Hammerhead

You may cast either half. That door unlocks on the battlefield. As a sorcery, you may pay the mana cost of a locked door to unlock it.

Apex Great White
If one or more tokens would be created under your control, create that many of those tokens plus one instead.

Apex Hammerhead
If one or more counters would be put on a permanent you control, that many plus one of each of those kinds of counters are put on that permanent instead.

Deepglow Skate

When this creature enters, double the number of each kind of counter on any number of target permanents.

Scaly Pete, Master Baiter

At the beginning of your upkeep, look at the top card of your library. If it's a land card, you may reveal it and put it into the battlefield tapped. When you do, create a 1/1 blue Fish creature token. If you control a 1/1 blue Fish creature token, create a 3/3 blue Shark creature instead.

Prime Speaker Zegana

Prime Speaker Zegana enters with X +1/+1 counters on it, where X is the greatest power among other creatures you control.
When Prime Speaker Zegana enters, draw cards equal to its power.

Mana Drain

Counter target spell. At the beginning of your next main phase, add an amount of {C} equal to that spell's mana value.

Cyclonic Rift

Return target nonland permanent you don't control to its owner's hand.
Overload {6}{U} (You may cast this spell for its overload cost. If you do, change "target" in its text to "each.")

Deserted Beach

This land enters tapped unless you control two or more other lands.
{T}: Add {W} or {U}.

Dreamroot Cascade

This land enters tapped unless you control two or more other lands.
{T}: Add {G} or {U}.

Backwaters of Port Clovis

Backwaters of Port Clovis enters the battlefield tapped unless you control a Fish, Mutant, Shark, or Leviathan.

{T}: Add one mana of any color in your commander’s color identity.

Glowcap Lantern

Equipped creature has "You may look at the top card of your library any time" and "Whenever this creature attacks, it explores." (Reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on that creature, then put the card back or put it into your graveyard.)
Equip {2}

Mikey & Leo, Chaos & Order

Whenever you put a counter on a creature you control, draw a card. This ability triggers only once each turn.

Keelbreaker, Bone Tyrant

Mutate {2}{W/G}{R}
Whenever this creature mutates, it gains indestructible and vigilance until end of turn.

{W}{R}{G}, {T}: Choose two —
• Put a shield counter on each of up to two target creatures.
• Destroy target artifact.
• Deal 3 damage to each creature.

Pir, Imaginative Rascal

Partner with Toothy, Imaginary Friend (When this creature enters, target player may put Toothy into their hand from their library, then shuffle.)
If one or more counters would be put on a permanent your team controls, that many plus one of each of those kinds of counters are put on that permanent instead.

Scenic Overbite

Destroy target artifact. Scenic Overbite deals 2 damage to that artifact's controller.

If a Shark you control dealt damage this turn, create a Map token.

Topography Tracker

When this creature enters, create a Map token. (It's an artifact with "{1}, {T}, Sacrifice this token: Target creature you control explores. Activate only as a sorcery.")
If a creature you control would explore, instead it explores, then it explores again.

Easter Egg Hunt

This spell cannot be countered.

Create a Map token, then discover 3

Three Visits

Search your library for a Forest card, put it onto the battlefield, then shuffle.