Extinction is really going to wreck havoc with this deck...
Ravenous Rats, originally from Urza's Destiny (though it might've come from Portal/Starter/whatever before that, can't remember.) They've seen a good amount of tournament and casual use, and that's not surprising.True. If that is not an issue for you, than disregard what I said.
6e had this 2 mana, 1/1 rat which caused the opponent to discard a card when it came into play. Not too shabby, IMO.