The Grim Adventures of Billy and Mandy (2001)

S1E18 Billy's Growth Spurt

Billy indulges in too much junk food, which leaves him feeling ill. To help, Grim concocts a remedy based on his grandmother's recipe, but instead, it causes a parasite to develop on Billy's back, which he whimsically names Yap-yap. As they continue their junk food binge, Mandy suggests using the antidote to counteract the original remedy. Eventually, Yap-yap, now resembling a mini Billy, separates from him, trying to escape the antidote. Ultimately, Yap-yap consumes the antidote, leading to his demise and freeing Billy from the unwanted guest.