Ghost of Merlin

Scooby-Doo 2 Monsters Unleashed
He did not appear in the movie as a monster. Although at The Coolsonian Criminology Museum, he was one of the few costumes seen in the DVD Special Features. All of the monsters from the series are supposed to be there.

Non-Cannon
In a alternate reality, Merlin wasn't really Zarko, he was actually real, and he transformed Daphne into The Little Mermaid), simply killing her in the process. Then she remained living underwater, then [[Shaggy Rogers|Shaggy and Scooby simply tricked Merlin into making them his slave. With another spinning transformation, it dodged it into a mirror spinning it near Merlin, simply killing him into pieces.