Rat Out

Up to one target creature gets -1/-1 until end of turn. You create a 1/1 black Rat creature token with "This token can't block."

Wick, the Whorled Mind

Whenever Wick or another Rat you control enters, create a 1/1 black Snail creature token if you don't control a Snail. Otherwise, put a +1/+1 counter on a Snail you control.
{U}{B}{R}, Sacrifice a Snail: Wick deals damage equal to the sacrificed creature's power to each opponent. Then draw cards equal to the sacrificed creature's power.

Blightbelly Rat

Toxic 1 (Players dealt combat damage by this creature also get a poison counter.)
When this creature dies, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Lab Rats

Buyback {4} (You may pay an additional {4} as you cast this spell. If you do, put this card into your hand as it resolves.)
Create a 1/1 black Rat creature token.

Plague of Vermin

Starting with you, each player may pay any amount of life. Repeat this process until no one pays life. Each player creates a 1/1 black Rat creature token for each 1 life they paid this way.

Song of Totentanz

Create X 1/1 black Rat creature tokens with "This token can't block." Creatures you control gain haste until end of turn.

Revenge of the Rats

Create a tapped 1/1 black Rat creature token for each creature card in your graveyard.
Flashback {2}{B}{B} (You may cast this card from your graveyard for its flashback cost. Then exile it.)

Admiral Brass, Unsinkable

When Admiral Brass enters, mill four cards.
At the beginning of combat on your turn, you may return target Pirate creature card from your graveyard to the battlefield with a finality counter on it. It has base power and toughness 4/4. It gains haste until end of turn. (If a creature with a finality counter on it would die, exile it instead.)

Gnawing Crescendo

Creatures you control get +2/+0 until end of turn. Whenever a nontoken creature you control dies this turn, create a 1/1 black Rat creature token with "This token can't block."

Don Andres, the Renegade

Each creature you control but don't own gets +2/+2, has menace and deathtouch, and is a Pirate in addition to its other types.
Whenever you cast a noncreature spell you don't own, create two tapped Treasure tokens.