Master Moo (known as Muu in Japan), more commonly known as Moo, is a fictional character in the Monster Rancher anime. He is the major antagonist in the entire show, and an extremely evil villain. He bears no redeeming qualities, and is shown as a personification of evil; his demonic form, his method of seducing monsters, and his ability to derive strength from hatred, all give him an undeniable similarity to the devil. Moo is a cruel and vicious sadist who commits his evil acts purely because of the enjoyment it gives to his dark soul.

Source: Wikipedia