Cat
A cat is a small cute creature that can breed twice every hour. Cats expand to fill all available space. (see catsplosion)
Cats are accurately depicted in Dwarf Fortress.
An amusing consequence of the 3rd edition D&D system is that a domestic housecat is capable of demolishing first level commoners in combat, doing a point of lethal damage with a single bite or claw attack and being able to make two claw attacks and one bite attack in a single round. First level human commoners generally have between 1 and 5 hit points.