3x17 - The Grim Adventures of Billy and Mandy

Billy's first cousin from the center of the Earth, Nergal Jr. goes to school for the first time, but he is worried that people may not like him. Soon enough, Junior becomes bullied by Sperg, with Miss Butterbean's allowance. Billy suggests Junior to use his shapeshifting abilities to take on the form of Miss Butterbean, so Junior can help him pass a math test. Junior likes the idea, but he morphs into Miss Butterbean to get revenge on Sperg instead. Sperg is seen living in the center of the Earth as Junior's pet.