Almost all of the monsters in the original Monster Manuel by Gary Gygax were "ideas" borrowed from other sources. In some cases he faced copyright problems, such as with the estate of Lovecraft, and so you do not see the Lovecraftian Mythos monsters in the 2nd edition onward. However, since Gygax himself borrowed most of his monsters, and since we see a virtual plethora of "beholder-like" creations elsewhere as well, I am going to guess the origination of the monster itself, it's likeness, is public domain. BTW, it is a LOVELY beastie... would be the pride and joy of every Dungeon Master's heart... make more like him :)