Here's a Mumbly short that wasn't entirely original.
Breakout Barney (John Stephenson) has escaped prison, bent on heading for Mexico. Enter Mumbly (Don Messick, also the voice of the prison warden), and the rest of this story will remind you of a couple of Droopy shorts from the 40's. Mumbly's boss, the Kojak clone Shnooker (Stephenson doing his Joe Flynn impersonation) is kept to a quick cameo.
Edit: 7/9/14: the video was deleted by YouTube as the poster's account was terminated. As soon as someone else uploads "Breakout Barney", we'll get it back up. Sorry.
How to explain Mumbly turning villain and joining the Really Rottens on Laff-a-Lympics the next year? As we've documented, Hanna-Barbera lost the rights to Dastardly & Muttley on a temporary basis due to a dispute with Heatter-Quigley, so Mumbly, who would be Muttley's exact double but for the blue fur, was conscripted, if ya will. However, the writers have never considered resolving the whole plotline, perhaps in the faint hope that viewers would forget his heroic past. Nuh-uh.
I'd say, do a DTV movie that would close the story of the Dread Baron and have it be revealed that Mumbly'd been working in deep cover all that time (this extended into the 80's), and would eventually turn on the Baron and bring him in. How hard is that?