Each of them is a bad influence on the rest.

Relentless Rats

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

Secret Lair Drop

Special Guests

If you provoke them, they grow malicious.

If you exalt them, they grow splendiferous.

Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to one Relentless Rats may become lethal if another Relentless Rats leaves the battlefield during that turn.

The last ability of Relentless Rats lets you ignore the "four-of" rule. It doesn't let you ignore format legality. For example, during a Bloomburrow Limited event, you can't add Relentless Rats from your personal collection.