just a guess but maybe it's a list of things that were featured prominently in products aimed at a wider audience, eg: movies like the recent one with Chris Pine and video games like BG3?
As best I recall, the recent movie involved a dragon and one or more liches. I think there was also a displacer beast, but it wasn't exactly significant in the film.
I'm not familiar with what products would have made umber hulks or yuan-ti prominent, but I can believe that they existed, particularly in the case of yuan-ti, which seem to have a lot of lore developed around them. A lot of lore in the present generally means a lot of products featuring them in the past. I'm just not sure how valuable it is to protect the name "yuan-ti" when the concept is so generic.
(The webcomic Rusty & Co. includes one character who is a snake person, named Yuan-Tiffany. She's a lamia, due to legal threats from Wizards of the Coast, but while the name of her species had to be changed, neither her character design ["snake person, no legs"; lamia nobles look like that too] nor her name was affected.)
Tanar'ri and baatezu are a weirder case, where the names are so worthless that even D&D itself prefers not to use them, and the concepts are obviously not subject to copyright or other protection. They originate in (or before) 1st edition D&D under the names "demons" and "devils", with no other name given, and a moral panic in the USA prompts TSR to rename those categories "tanar'ri" and "baatezu" in the second edition (without changing the monsters - having a monster called a "succubus", an evil spirit hailing from "the Abyss" who exists to lead mortals into evil by having sex with them, is fine; labeling that monster a "demon" is not). In third edition, they're back to being "demons" and "devils", with the names "tanar'ri" and "baatezu" being mentioned, but clearly positioned as obscure lore about the universe. And that's how it's been ever since.
