Typhoid Rats

Deathtouch (Any amount of damage this deals to a creature is enough to destroy it.)

Burglar Rat

When this creature enters, each opponent discards a card.

Drainpipe Vermin

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

Cling to Dust

Exile target card from a graveyard. If it was a creature card, you gain 3 life. Otherwise, you draw a card.
Escape—{3}{B}, Exile five other cards from your graveyard. (You may cast this card from your graveyard for its escape cost.)

Gnat Miser

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

Ink-Eyes, Servant of Oni

Ninjutsu {3}{B}{B} ({3}{B}{B}, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand tapped and attacking.)
Whenever Ink-Eyes deals combat damage to a player, you may put target creature card from that player's graveyard onto the battlefield under your control.
{1}{B}: Regenerate Ink-Eyes.

Snow-Covered Swamp

Crypt Rats

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

Cremate

Exile target card from a graveyard.
Draw a card.

Rat Colony

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