Monster Mutt appears in "A Scarey Duel with a Cartoon Ghoul", an episode of The New Scooby-Doo Mysteries.
About Monster Mutt
He is an in-universe cartoon villain, the enemy of Hero Hound. Monster Mutt is a bipedal dog with human hands. He wears a green Dynomutt-style cape, a purple domino mask and purple boots with green soles.
Dimwittie the studio guard (really a master spy) brought Monster Mutt to life as a disguise in order to steal rocket secrets.