Scooby Dum was only used in this one right?

Scooby-Dum appeared later on in a different series where he and Scooby-Doo and Scooby-Dee all appear in the same episode. That episode is called "The Chiller Diller Movie Theater". Another episode is called "Vampire Bats and Scaredy Cats". These two episodes aired during the 1977 series SCOOBY's ALL-STAR LAFF-A-LYMPICS.

A total of 40 episodes ran for three seasons, from 1976 to 1978, on ABC. Sixteen episodes were produced as segments of The Scooby-Doo/Dynomutt Hour (aka The Scooby-Doo/Dynomutt Show) in 1976, eight episodes were produced as segments of Scooby's All-Star Laff-A-Lympics in 1977 and sixteen episodes were produced in 1978.

The 16 episodes that aired in 1978 were released on DVD as "Scooby-Doo, Where Are You? Season Three". The eight episodes, however, that aired in 1977 have yet to be released on DVD and two of those eight episodes feature the Scooby-Dum character in the episodes I mentioned at the top of this post.

Scooby-Dum's first two appearances, "The Gruesome Game of the Gator Ghoul" and "The Headless Horseman of Halloween" as you know appeared on the 1976 series SCOOBY-DOO/DYNOMUTT.