Razortooth Rats

Fear (This creature can't be blocked except by artifact creatures and/or black creatures.)

Chittering Rats

When Chittering Rats enters the battlefield, target opponent puts a card from their hand on top of their library.

Rats' Feast

Exile X target cards from a single graveyard.

Ravenous Rats

When Ravenous Rats enters the battlefield, target opponent discards a card.

Nezumi Ronin

Bushido 1 (Whenever this creature blocks or becomes blocked, it gets +1/+1 until end of turn.)

Swamp

Okiba-Gang Shinobi

Ninjutsu {3}{B} ({3}{B}, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand tapped and attacking.)
Whenever Okiba-Gang Shinobi deals combat damage to a player, that player discards two cards.

Relentless Rats

Relentless Rats gets +1/+1 for each other creature on the battlefield named Relentless Rats.
A deck can have any number of cards named Relentless Rats.

Bog Rats

Nezumi Cutthroat

Fear (This creature can't be blocked except by artifact creatures and/or black creatures.)
Nezumi Cutthroat can't block.

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.

Blightbelly Rat

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