What sick, twisted mind would turn this guy evil. Its madness I tell you, madness!: Tonto
The Cookie monster is a being from Sesame Street, and a minion of the evil Dr. Doofemsmirtz.
While the show, Sesame Street, exists as just that in the KRspaceT universe, this dark version of the Cookie monster was created by Dr, Doofemsmirtz in order to destroy the universes sweet supply. Kiki and Gwen managed to destroy it, and recover the stolen cookies, or most of them anyway.