Not to be confused with the puppet Mad Dog in the original Dog City special, Mad Dog is a slobbering, wolf-like crook, who normally communicates only through snarling and rapid mauling. When conked on the head, however, as typically occurred at least once per episode, Mad Dog would suddenly become articulate and refined, quoting Shakespeare or some other noted poet or philosopher (similar to the Mad Dog from the special). How Shakespeare's utterances were known, undiluted, in a world of dogs remained one of the series' mysteries.
In the first season, Mad Dog's fur was dark green. His fur was later changed to brown in the next two seasons.