The Grim Adventures of Billy and Mandy (2001) : 4x2

When Billy and Mandy's school is preparing for a big game, Mindy entrusts the care of the school's mascot, Fluffy Cat, to Billy, to protect him from being kidnapped by the rival team, the Booty Heads. At Billy's house, Fluffy Cat is a disaster, so Billy's mom throws him into the basement, where the cat gets inside Grim's trunk, absorbing its powers. A giant Fluffy Cat appears just before the game wreaking havoc.