Yes, you can trigger the killer rabbit's appearance by killing lots of regular bunnies (save periodically, in case he is too tough to handle, and there is nothing nearby to climb or jump onto).
At level 15 my creature could not do any damage, but did hold the killer rabbit's attention (he may have been able to do better with a ranger or mage head, though). I think the demon is generally considered to be the best summon for combat.
