Early on, creatures learned skills just like players did. In the alpha, this led to wolves who hunted bunnies, and some bunnies the bunnies escaped and learned a little, and eventually there were both dire wolves (their name actually changed to "wise elder wolf" or something) and vorpal bunnies. These latter ones killed many unsuspecting players in the alpha, and we had to remove the creature learning because it was bad UX> We left in a vorpal bunny template in the launch game, however, as an Easter Egg.

That would explain the killer rabbits in FF Online.

Or at least magic-resistant, human-transmittable chronic wasting disease. Or deer ticks.