The real "killer rabbit" of the Magic world is the squirrel. Some squirrel-themed cards, such as "Deranged Hermit" and "Squirrel Nest" have seen extensive Tournament Play.
Hyalopterous Lemure. The artist didn't know what a "lemure" is (a ghost) so he drew a "lemur" instead (a fuzzy primate).
Also, Jackalope Herd. A bunch of bunnies with antlers, amazing combat stats for their mana cost, and a drawback (returning to your hand when you play another spell) that can be turned into an advantage fairly easily.
Perhaps most infamous is the Vizzerdrix, a Starter-deck exclusive card that has an awful P/T to mana cost ratio and no abilities. However, it is a 6/6, meaning that it can defeat a typical dragon and kill a Physical God (the players) in four hits.
There's a level 2 monster that's basically a rabbit with a switchblade, where the Bad Stuff is "you die" (yes, a literal Killer Rabbit; not surprising since he's actually Bun-bun, the Sluggy Freelance character. One time in six, it turns into a level 15 monster, as "That One Rabbit from That One Movie", and it's too late to get help.
The Munchkin Bites! expansion has 17th-level monster The Evil, also from Sluggy Freelance. For those not versed in the webcomic, it's an adorable cute kitten. It steals a level from you just by meeting it.
The Duck of Doom. You should know better than to pick up a duck in a dungeon!
Killer Bunnies and the Quest for the Magic Carrot
Yu-Gi-Oh! gives us Mokey Mokey. It looks cute and harmless, and it is, but with this card present on the field at the same time, you will not want it to get angry.