Beholders were specifically designed for DnD, so they can be used by Wizards of the Coast to bully you (no, they are not fighting copyright infringement. They are just bullying you, however the law backs them up, showing that justice is not always just). Same goes for gnolls and Sahuagin, and I couldn't even bother anymore to look up the Mind Flayer (which I did also find in Final Fantasy I (for PlayStation), under a different name, but I guess that's rather a miss-translation than a copyright issue avoidance).
Like MaxSMoke777 said, the majority of the monsters you'll find in the monsters manual for DnD (any edition) are directly copied from folklore and mythology and some of them even from religions that still have followers today (like Tarasque, which came from a French Christian legend). Trolls regenerating can be a hard one.
Trolls themselves are in the public domain due to being part of age old folklore. I looked them up, but could not find mention of regenerating properties. Now I did also find Trolls in the Wild Arms series, who have a free healing action at the end of every turn in combat, which makes me think that was heavily inspired on the regeneration properties the DnD-variant is famous for, and I doubt Wizards of the Coast were ever paid royalties for the examples I just named.