Dick Dastardly is a character from Wacky Races (1968) and Dastardly and Muttley in Their Flying Machines (1969) by Hanna-Barbera. He has a sidekick, the dog Muttley.
Muttley and Dick Dastardly are replaced by Dread Baron and Mumbly in the animated series Laff-a-Lympics (1977) and the film Yogi Bear and the Magical Flight of the Spruce Goose (1987).
Mumbly the dog looks like Muttley, the dogs was voiced by Don Messick, who played a lookalike dog in the film Hey There, It's Yogi Bear (1964).
One possible explanation is that since Wacky Races was co-produced with Heatter-Quigley Productions, perhaps Hanna-Barbera did not have the rights to Muttley and Dick Dastardly.
Interestingly, around the same time as the film, the duo appears in the animated series Yogi's Treasure Hunt (1985-1988).
In Laff-A-Lympics #13, (March 1979) published by Marvel Comics, written by Mark Evanier and illustrated by Jack Manning, Scott Shaw and Dan Spiegle, reveals that Dick and Dread are twin brothers.
Apparently, that issue was resolved and Muttley and Dick were never replaced in other works.