Wonder-Dog is the superhero alter-ego of Ace Hart, who makes his first (and last) appearance in the Dog City episode "Ya Gotta Have Hart." Ace is starring in his own comic book, but to the surprise of both Ace and Eliot Shag, the editor made some last minute alterations, turning the script into a superhero origin story.
Ace confronts Baron Von Rottweiler at his laboratory, where the baron has been experimenting with radioactive chewies. Leon sprays Ace with the radioactive water intended for Rottweiler's Sci-Fido. Ace grows enormously muscular, and according to the narration, "and on that fateful day, Ace Hart became... Wonder-Dog!" Both Rottweiler and Ace are nonplussed by this development.
In a parody of the famous Superman radio show opening, the announcer further states that Wonder-Dog is faster than a speeding bulldog, more powerful than a pack of pumped up pups, and able to leap tall fences in a single bound.