If I understand correctly, they only own copyrights for the lore, not the names and appearance. Also product identity so they can't be used by other tabletops, but other media can use them. For example, the game Demon's Souls have Mind Flayers, they are called Mind Flayers and look the part. I doubt From Soft and/or Sony are paying royalties for such minor enemy.

I think this case is different from the Beholder which seemingly they own complete rights since they made Tibia change their Beholder to something distinct.