Mezzodaemons and Nycadaemons are in Appendix E of the DMG and show up on its encounter tables. They're also in Fiend Folio, as has been noted upthread.As far as I know, Yugoloths themselves have never shown up in core D&D in any edition.
OK, but that means Gygax's PHB names for the planes, and the classic Monster Manual descriptions for their inhabitants. (Eg no Blood War.)Bingo, if there needs to be a default cosmology, which I don't think there needs to be, The Great Wheel (Planescape without the dressing) makes the most sense
These sources don't have the Planescape backstory, however. Nor the moral relativist/"belief" oriented outlook that seems to be characteristic of Planescape. For example, there is nothing at all in these books to suggest that a devil and an astral deva might sit and drink together in a bar.That's not true; 1st Ed PHB, DMG, MM, MM 2, Deities & Demigods, Manual of the Planes.
the only things Planescape added were Sigil and the factions.
And a huge amount of backstory about the outer planes - for instance, that daemons have some special hatred for gods; the Blood War; that demons and devils have a secret origin related to the "yugoloths"; etc.Planscape just added some window dressing to the core D&D multiverse
Your characterisation of the 4e MM is highly disputable. Look at the entries for goblins (and hobgoblins and bugbears), for orcs, and for gnolls - you'll see far more lore than in the AD&D MM. Look at the entry for demons and devils - you'll see respectively a history of the Abyss, and a tourist's guide to the Nine Hells. Look at the entry for spiders and you'll learn that Lolth was once a god of fate. The only thing missing from the bulette entry is their love of hobbits, which is probably excusable in a game world in which halflings are river folk rather than burrowers.What we don't want is a bland MM like the 4e one with virtually no lore for each monster
There is very little substance to this oft-repeated claim about the 4e MM, as came out on a thread late last year comparing the 4e entries to the 2nd ed Monstrous Manual entries.