Its not a a problem i actually encountered myself, but few people in the forums experienced it so i thought it would be a good idea to inform you about it.

When people have encountered the rare killer rabbit, it had a name attached to it saying "entity.killerbunny.name"

Examples:
1.
http://www.minecraftforum.net/forums/minecraft-discussion/recent-updates-and-snapshots/2158626-i-found-the-killer-rabbit-of-caerbannog

2.
http://www.minecraftforum.net/forums/minecraft-discussion/recent-updates-and-snapshots/2154174-killer-bunny-entity-title-bug