The Bunny is an NPC added in the 1.0.4 patch. It can be seen on many of the Terraria logos, and spawns on grass blocks without backgrounds. It is normally a friendly creature with 5 HP, which is damaged by monsters, and is incapable of dealing damage. However, if you throw Vile Powder on it, or there is a Blood Moon, it turns into a Corrupt Bunny.
Ficheiro:Anomaly.png Anomaly.Bunnies do not move at night time, instead opting to stand in one place until sunrise.