The Grim Adventures of Billy and Mandy (2001)

S4E2 He's Not Dead, He's My Mascot

As the school gears up for an important game, Mindy asks Billy to look after the beloved mascot, Fluffy Cat, to keep him safe from the rival Booty Heads' plans to steal him. However, chaos ensues when Fluffy Cat wreaks havoc at Billy's home, prompting his mom to toss him into the basement. There, the mischievous cat stumbles into Grim's trunk and absorbs its magical powers. Just before the game kicks off, a colossal Fluffy Cat emerges, unleashing mayhem on everyone involved.