The Grim Adventures of Billy and Mandy (2003)

As Billy and Mandy wish Mr. Snuggles, Billy's 10-year-old hamster, a happy birthday, The Grim Reaper arrives to take the aged rodent back to the netherworld. Mandy strikes a deal with the Grim Reaper; if he wins, he can keep Mr. Snuggles; but if she wins, she and Billy get to keep him. A super confident Grim Reaper adds, "If I lose, I'll be your best friend forever." Unfortunately for Grim, Mandy is more formidable than he had bargained for.