The Grim Adventures of Billy and Mandy (2001)

S1E12 Grim or Gregory?

On Halloween night, Billy, Mandy, and Grim set out for trick-or-treating. Grim approaches a woman he believes is Atrocia, the star of a Halloween marathon, but soon realizes his error. However, the woman confuses Grim for her son, Gregory, and brings him home instead. Now in Gregory's shoes, Grim finds himself without a stomach. Meanwhile, Billy misidentifies Gregory as Grim, prompting him and Mandy to take him along on their escapades. Ultimately, they return Gregory to his mother while Grim rejoins Billy and Mandy.