Most people keep on focusing on monsters published in D&D sourcebooks, and I really think that's a dead end, especially with regards to the more obscure D&D monsters. Rich never said that Monster-San existed in D&D before he put him in the story; he just said that he existed. And he's been working for a long time at making the comic accessible to folks unfamiliar with D&D: When Monster-San is eventually revealed, it's going to be something that most of the readership, not just the D&D players, will be able to go "Oh, OK, it's <one of those>!". Now, some monsters that pre-existed D&D have made it into the rulebooks, but those are mostly going to be found in the first volume, and I think we've pretty well settled that none of those are a good fit. So I think it's likely that he's not in any D&D rulebook at all.