[Edi]

In any case it may pay to be prudent and avoid serious conflicts. Beholder as a name is interesting in that it's actually a word of the English language, so you can't copyright that, but the concept of the floating spherical monster with magic eyes that is called a beholder may in fact be specific enough to be copyrighted.

Mind flayers I'm not sure about, but that could depend a lot on how Lovecraftian their origins are. In Dominions they are mainly called starspawn and it's the same in CoE3. Illithid is a TSR/WOTC name, which appears in Dominions and it is more problematic.