Mad Dog is the burly but stupid henchman to Bugsy Them in Dog City. He is unable to resist obeying such commands as "sit" and "stay", thanks to spending two (I mean fourteen) years in obedience school.
Throughout the show, Mad Dog makes only grunts and squeals. However, when Bugsy proclaims he doesn't even know the meaning of lost when he has nearly been defeated by Ace Yu. Mad Dog proceeded to give definitions of "lost", complete with examples, much to the astonishment of Laughing Boy and Scruffy.
In the later spinoff series, there was a different Mad Dog, who served as henchman to Bugsy Vile. The original Mad Dog puppet was recycled as Bowser for the Muppet framing sequences. Prior to that, however, Mad Dog was amongst the dogs in the pet detention center in The Muppets at Walt Disney World.