They are still out there waiting to be unearthed! Above is a 1970s dimestore bulette that I recently rescued from the bottom of a bin of plastic dinosaurs at a vintage toy store. This is the Chinese-manufactured toy upon which the D&D monster was based. I had several sets of these 'Prehistoric Monsters' when I was a kid, well before I heard of D&D. We called this one Blaster, and it had two powers ---- it could "explode" itself, and travel through the ground, similar to a bulette but more like phasing.
Holmes himself owned several of these bulettes. For a picture, see this post ---
http://zenopusarchives.blogspot.com/2017/09/tales-of-peril-dust-jacket-flaps.html?m=1
At North Texas RPG Con last weekend I had the pleasure of meeting Darlene, an artist for TSR who did the original World of Greyhawk maps in the 1983 boxed set. I asked her to sign my original copy of the WoG Glossography, one of the books from the set. I picked this because part of her beautiful map is shown on the cover, but she relayed that she had also designed the graphics. She stared at title page for a few seconds, remembering her work.
In addition I bought a copy of her 1982 card game, Jasmine: The Battle for the Mid-Realm, which you can see here on BGG. On the bottom side of the clear box for the game I spied a Skull Mountain-esque picture I'd never seen before on one of the cards. See above.
Darlene has a page about Idshoii on her blog, which verifies that it's not just a castle, but a carving at the top of a mountain:
"Ildshoii Castle, carved from shiny black obsidian rock, rose high above a crown of volcanos. The dark castle was the domain of Melantha, the ruler of Medrylthorn ..."
Update: Here is Darlene's own intro page for the game:
