You know, I don't see what's to keep anyone from taking the basic Beholder, Mind Flayer or Umber Hulk stats from the Monster Manual, then attach a whole new decription/background to the creature and call it something new. Heck, it would go a long way towards being original.
Can't use the Beholder? Ok, then how about the Krystalix, a floating spheroid creature seemingly made of crystal, with multi-colored gems for eyes, serted all over it's round body?
Can't use the Mind Flayers? Then try the Psi-Wraiths, a race of mysterious beings of psionic energy who can only interact with the physical world via their "DarkSuits", Vorlon-like costumes which hide their forms from mortal eyes.
Can't think of anything appropriate for Umber Hulks atm, though.