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.")

Groundchuck & Dirtbag

Trample
Whenever you tap a land for mana, add {G}.

Michelangelo, On the Scene

Trample (This creature can deal excess combat damage to the player it's attacking.)
Michelangelo enters with a +1/+1 counter on him for each land you control.
When Michelangelo dies, return this card to your hand.

Mikey & Mona, Mutant Sitters

When Mikey & Mona enter, choose one or both. Each mode must target a different player.
• Target player chooses a creature they control and puts two +1/+1 counters on it.
• Target player returns a creature or land card from their graveyard to their hand.

Michelangelo, Improviser

Sneak {2}{G}{G} (You may cast this spell for {2}{G}{G} if you also return an unblocked attacker you control to hand during the declare blockers step. He enters tapped and attacking.)
Whenever Michelangelo deals combat damage to a player, you may put a creature card and/or a land card from your hand onto the battlefield.

Mona Lisa, Science Geek

Reach
{T}: Add X mana of any one color, where X is Mona Lisa's power.

Michelangelo, Game Master

Disappear — At the beginning of your end step, if a permanent left the battlefield under your control this turn, put a +1/+1 counter on Michelangelo.

Leatherhead, Swamp Stalker

Trample
Leatherhead enters with a hexproof counter on her.
Whenever Leatherhead deals combat damage to a player, you may remove a counter from her. When you do, destroy target artifact or enchantment that player controls.

Michelangelo, Mutant BFF

Each creature you control with a counter on it can't be blocked by more than one creature.
Whenever Michelangelo enters or attacks, 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.")

Rocksteady, Crash Courser

Rocksteady can't be blocked by more than one creature.
Boars you control can't be blocked by more than one creature.
Forestcycling {2} ({2}, Discard this card: Search your library for a Forest card, reveal it, put it into your hand, then shuffle.)

Ferrafor, Young Yew

When Ferrafor enters, create a number of 1/1 green Saproling creature tokens equal to the number of counters among creatures target player controls.
{T}: Double the number of each kind of counter on target creature.

Michelangelo, the Heart

Trample
Raid (the Fridge) — At the beginning of your second main phase, if you attacked this turn, put a +1/+1 counter on target creature and create a Food token.
Partner—Character select (You can have two commanders if both have this ability.)

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.

Mona Lisa, Ever Adaptable

Whenever a player casts a creature spell, you 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.")

Leatherhead, Iron Gator

Trample, haste
Whenever Leatherhead attacks, put two +1/+1 counters on each creature you control.

Rocksteady, Mutant Marauder

Partner with Bebop, Skull & Crossbones (When this creature enters, target player may put Bebop into their hand from their library, then shuffle.)
Trample
Whenever another nontoken creature you control enters, put a +1/+1 counter on target creature.