The Grim Adventures of Billy and Mandy (2001)

Season 3: Episode 30

When Billy has a loose tooth he cannot wait for the Tooth Fairy to appear to give him money. Mandy, who does not believe in the Tooth Fairy, asks Sperg to end Billy's waiting by knocking his tooth off. Sperg's punch leaves Billy unconscious, who has a dream about being in the wild west, where the Tooth Fairy has taken his tooth. Mandy challenges the Tooth Fairy to a showdown to recover the tooth, and she comes out victorious just as Billy awakens. Billy does find money under his pillow, while the aged Tooth Fairy watches through the window.