In a nod to old-school Plague Rat decks, Wizards choose to put this upgraded version in the Fifth Dawn set. The card immediately gave birth to innumerable casual decks featuring a easily-guess-able theme.
If you come up against a Relentless Rat, it is a safe guess there are more to come!
Particular foes of Relentless Rats include Maelstrom Pulse, Thought Hemorrhage and Haunting Echoes.
One of the best MTG-related things I've come across in a while is the song "Relentless Rats" performed by Jack Bowen during the credits of Episode 5 of Noobie and the Dood.
Fast forward to 6:20 if you just want to hear the song. The entire episode, however, is vaguely Relentless Rats themed.
Noobie and the Dood aren't making too many more episodes, however you can hear more of Jack Bowen's amazing musical talent on the "Dredging For Humor" podcast found here.