Stegmutt is a stegosaurus, who was originally a duck who was turned into a dinosaur. Stegmutt first appeared in the two "Justice Us, Justice Ducks" episodes (44 and 45 in production order), but due to the episodes airing out of order his introduction episode, "Jurassic Jumble" (28th in production order) aired a week later. He also appears in "Darkwing Doubloon," his only other episode.
Background
He once was the assistant of a mad scientist named Dr. Fossil. Dr. Fossil in one of his experiments turns Stegmutt into a dinosaur using the Retro-Evolution Gun. Eventually, Gosalyn and Honker convince Stegmutt that Dr. Fossil is evil and he turns against him. Due to Stemutt’s friendly nature he becomes part of the "Justice Ducks."