Stegmutt is rather dim-witted but has a good heart. He likes helping Darkwing Duck and his friends. He also has a strong dislike for fire, and when he finds the source, he puts it out by slamming it onto the ground. He became a member of the Justice Ducks and helped Darkwing defeat the Fearsome Five. He's good-natured, simple, and honest to a fault.
It's unclear if Stegmutt's simple-minded nature is his own, or is the result of his transformation at the hands of Dr. Fossil. After all, Stegosaurus are famous for having "a brain the size of a walnut." His low-range job as a janitor doesn't help to clarify matters.
He once was the assistant of a mad scientist named Dr. Fossil. In one of his experiments, Dr. Fossil transformed 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.
Before Stegmutt was a dinosaur, he used to be a duck. He also used to work as a janitor at the Natural History Museum, where he continues to live in hiding as one of the displays. Due to Stegmutt's friendly nature, he becomes part of the "Justice Ducks." He is also a hot dog vendor in his spare time, a pathologically honest one who chases Darkwing Duck throughout both halves of "Just Us Justice Ducks" to return his spare change.
- The spikes Stegmutt uses to open his soda cans are called a "Thagomizer," a term coined in Gary Larson's "The Far Side" cartoon strip for the previously unnamed feature of the Stegosaurus (yet another inadvertent tribute to Larson's classic comic in DwD).