What a Cartoon! S02 E04 The Grim Adventures of Billy & Mandy: Meet the Reaper

Billy and Mandy are paid a visit by the Grim Reaper when he comes to collect the soul of Billy's hamster, Mr. Snuggles. Mandy then makes a bet with him in the form of a game: if Grim loses, he lets them keep Mr. Snuggles AND become their "best friend".