Tanar'ri and baatezu are, even within full-copyright Official D&D materials, better known as "demons" and "devils", and I have a hard time imagining that any third-party material would want to use the protected names.
Beholders are iconic, and distinctive to D&D, and gauths are derivative of them. There are several other beholder-lites; why aren't spectators protected?
Illithids and mind flayers are the same thing, always fairly iconic and more so now that Baldur's Gate 3 is devoted to them, and githyanki and githzerai are part of the lore developed around them.
Yuan-ti are snake people. They're not distinctive at all; the film Conan the Barbarian has a villain who does a ritual to turn into a giant snake. The general concepts of snake people, snakier people, and evil snake gods are too popular and generic to be protected, but I guess you should call them something other than "yuan-ti".
(Fun side note: the name "yuan-ti" looks like it's obviously Chinese. You can find questions about this all over the Chinese internet, since there's no obvious way to assign meaning to the name. The answer appears to be that the guy who made up the name knew nothing about Chinese and it's all a big coincidence.)
Umber hulks are large insectoid creatures that burrow underground. I'm not sure why they're considered so important, but they're more distinctive than carrion crawlers.