Who turns out to be an unregistered Animagus in Harry Potter and the Goblet of Fire?

The unregistered Animagus in Harry Potter and the Goblet of Fire is Bartemius "Barty" Crouch Jr.

He's able to transform into a dog, and his father, Bartemius Crouch Sr., uses the Imperius Curse to make him masquerade as Mad-Eye Moody throughout the entire Triwizard Tournament.