I have a good story: I was in Lake Geneva on my way (big roadtrip from CA, long story) to GenCon 2000 and I stopped by the local game shop. Who was working the counter? None other than Ernie Gygax! I kid you not. So we got to talking and Ernie said he'd lived in LA for awhile in the 80's when he was a consultant on the DnD cartoon. His job was to make sure everything in the cartoon matched up to the game, such as monsters. Well I had recently seen some bootlegs of the old cartoon so I asked him, "Well what about DemoDragon?" (DemoDragon appeared in an episode wherein the kids were trapped in a fortress under seige by Venger who summoned DemoDragon to break down the doors. DemoDragon was big as the fortress itself and looked like a cross between DemoGorgon and Tiamat: 2 dragon heads, 4 cloven hooves, and tentacles.) Not in the MM, Fiend Folio or anywhere! So stumped Ernie Gygax! He had let it slide... To this day I doubt there's stats for DemoDragon, but there should be, he kicked Venger's butt!
Well that's good news. Great monster. The whole DnD cartoon thing might make for a good theme for an adventure in Dungeon...
Ernie was non-plussed but he still invited me to his weekly pick-up game that evening, which was great time in itself.
There was a Creature Catalog incarnation of him at EN World (but the link's broken right now), so I decided to do my own stats for him: a cross between a 13-tentacled chole dragon (has the tentacles and lives in the Abyss), dread linnorm (has the two heads that breathe fire and cold), and an aspect of Demogorgon (has the two heads and tentacles).