Muck Rats

Bog Rats

Kuro's Taken

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

Carrion Rats

Whenever this creature attacks or blocks, any player may exile a card from their graveyard. If a player does, this creature assigns no combat damage this turn.

Gutter Skulk

Drainpipe Vermin

When this creature dies, you may pay {B}. If you do, target player discards a card.

Gobhobbler Rats

Hellbent — As long as you have no cards in hand, this creature gets +1/+0 and has "{B}: Regenerate this creature." (The next time it would be destroyed this turn, instead tap it, remove it from combat, and heal all damage on it.)

Chittering Rats

When this creature enters, target opponent puts a card from their hand on top of their library.

Gnat Miser

Each opponent's maximum hand size is reduced by one.

Burglar Rat

When this creature enters, each opponent discards a card.

Nezumi Bladeblesser

This creature has deathtouch as long as you control an artifact.
This creature has menace as long as you control an enchantment. (It can't be blocked except by two or more creatures.)

Disease Carriers

When this creature dies, target creature gets -2/-2 until end of turn.

Rat Colony

This creature gets +1/+0 for each other Rat you control.
A deck can have any number of cards named Rat Colony.

Mukotai Ambusher

Ninjutsu {1}{B} ({1}{B}, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand tapped and attacking.)
Lifelink

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.)

Totentanz, Swarm Piper

Whenever Totentanz or another nontoken creature you control dies, create a 1/1 black Rat creature token with "This token can't block."
{1}{B}: Target attacking Rat you control gains deathtouch until end of turn.

Earsplitting Rats

When this creature enters, each player discards a card.
Discard a card: Regenerate this creature.

Dirty Wererat

{B}, Discard a card: Regenerate this creature.
Threshold — As long as there are seven or more cards in your graveyard, this creature gets +2/+2 and can't block.

Nezumi Cutthroat

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

Crypt Rats

{X}: This creature deals X damage to each creature and each player. Spend only black mana on X.