Forest

Sandcrafter Mage

When this creature enters, bolster 1. (Choose a creature with the least toughness among creatures you control and put a +1/+1 counter on it.)

Song of Serenity

Creatures that are enchanted can't attack or block.

Sentry Bot

Flash
This spell costs {1} less to cast for each creature attacking you.
When this creature enters, you get {E} for each creature attacking you.
At the beginning of combat on your turn, you may pay {E}{E}{E}. If you do, put a +1/+1 counter on each creature you control.

Securitron Squadron

Squad {3} (As an additional cost to cast this spell, you may pay {3} any number of times. When this creature enters, create that many tokens that are copies of it.)
Vigilance
Whenever a creature token you control enters, put a +1/+1 counter on it.

Stag Beetle

This creature enters with X +1/+1 counters on it, where X is the number of other creatures on the battlefield.

Sumala Rumblers

Sumala Rumblers's power is equal to the number of creatures you control.
Myriad (Whenever this creature attacks, for each opponent other than defending player, you may create a token copy that's tapped and attacking that player or a planeswalker they control. Exile the tokens at end of combat.)

Secure the Wastes

Create X 1/1 white Warrior creature tokens.

Plains

Scourge of Skola Vale

Trample
This creature enters with two +1/+1 counters on it.
{T}, Sacrifice another creature: Put a number of +1/+1 counters on this creature equal to the sacrificed creature's toughness.

Shrewd Storyteller

Survival — At the beginning of your second main phase, if this creature is tapped, put a +1/+1 counter on target creature.

Soltari Visionary

Shadow (This creature can block or be blocked by only creatures with shadow.)
Whenever this creature deals damage to a player, destroy target enchantment that player controls.

Siegehorn Ceratops

Enrage — Whenever this creature is dealt damage, put two +1/+1 counters on it. (It must survive the damage to get the counters.)

Satyr Grovedancer

When this creature enters, put a +1/+1 counter on target creature.

Sage of the Skies

When you cast this spell, if you've cast another spell this turn, copy this spell. (The copy becomes a token.)
Flying, lifelink

Simplify

Each player sacrifices an enchantment of their choice.

Space Marine Devastator

Squad {2} (As an additional cost to cast this spell, you may pay {2} any number of times. When this creature enters, create that many tokens that are copies of it.)
Grav-cannon — When this creature enters, destroy up to one target artifact or enchantment.

Shire Shirriff

Vigilance
When this creature enters, you may sacrifice a token. When you do, exile target creature an opponent controls until this creature leaves the battlefield.

Skyshroud War Beast

Trample
As this creature enters, choose an opponent.
Skyshroud War Beast's power and toughness are each equal to the number of nonbasic lands the chosen player controls.

Snake of the Golden Grove

Tribute 3 (As this creature enters, an opponent of your choice may put three +1/+1 counters on it.)
When this creature enters, if tribute wasn't paid, you gain 4 life.

Seeker of Sunlight

{2}{G}: This creature explores. Activate only as a sorcery. (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.)

Soul's Attendant

Whenever another creature enters, you may gain 1 life.

Spinner of Souls

Reach
Whenever another nontoken creature you control dies, you may reveal cards from the top of your library until you reveal a creature card. Put that card into your hand and the rest on the bottom of your library in a random order.