Fred as a villain?

Okay, this is a weird question. I distinctly remember having seen an episode of one of the later Scooby Doo series as a child in which Fred was the man behind the mask.

Whatever series this was, it did not feature Fred, and possibly did not feature one of the girls. I'm thinking it may have been "The New Scooby and Scrappy-Doo Show" (1983), which featured only Shaggy, Scooby, and Daphne, but it could have been something else.

From what I remember, it may have had something to do with a jewel robbery. I think Fred was trying to get back at the gang for kicking him out or something. I don't know. It is a very, very dim memory.

But the thing is...I've looked for things online about it, and I can't find any information. The whole thing struck me as odd as a child, and I never saw the episode again. Did this really exist? I'm almost sure it did, but maybe I dreamed it. If anyone can help, I'd appreciate it.