"Mange" or "The Secret File of the Ronin Rover"

The Secret Files of the Spy Dogs – Season 1

There is a rumored bad dog in town. The SpyDogs are agast! He's not in their databanks, so Ralph offers to go check him out. But when Ralph and Mitzy finally confront the dog, Ralph can't do anything, because the "bad dog" is his brother, Mange.