I have a question for anyone with experience regarding Open Gaming Licenses--particularly with D&D. I know for 5th edition they have updated it. I've been developing my own RPG for a long time with unique mechanics, setting, creatures, etc. That being said, I have leaned on the D&D bestiary, because it's superb. Does anyone know how they are handled?

Loads of critters are owned and not shared by WotC, such as Illithids (even though they're clearly Cthulhu-derived) and Beholders.

 

On the other hand, most of the monster manuals tend to just be things from mythology, and those obviously can't be trademarked. Generally speaking, though, if you can't find a mythological (or fairy-tale) instance of a particular beast, it's best not to use it.