4x16 - The Grim Adventures of Billy and Mandy

After being utterly bored of watching the same movie 60 times in a row, Grim takes Billy and Mandy to "Underworld Video" where they rent "The Legend of Chupacabra". While Billy watches the tape, a small, pink chupacabra emerges from his television and Billy decides to keep it as a pet, naming it Daisy. Mandy and Grim convince Billy that he is inept in taking good care of pets, and they return the blood-sucking chupacabra to the videotape.