Beholders are just plain nasty critters, and they are about as iconic as dragons when it comes to instant recognition of extreme danger.
Any sort of floating ball of eyes will have the player characters diving for cover in a flash, with a quick assessment of all available exits and readying the best assortment of ranged weapons they can muster.
Beholders are floating arsenals of magical mayhem, the following variant breeds are something new and unusual to throw at your players, because Beholders, I think, should be quite individual and distinctive creatures, no two exactly alike.
Of course, each and every Beholder firmly believes it is the One True Form of Beholder Kind, and all others are but mutant inferior breeds.. Such is the arrogance of the bloated hovering noggins.
First up on the menu, the Beholder Mind Eye, even more unusual than your average eye tyrant in that it has no eyes.. yep, none.
The Mind Eye compensates for this by developing awesome mental powers, which, while not so lethal in a fair fight, become wicked to witness while dueling with adventurers in a lair chock full of pits, walls of spikes and kissing maiden traps.
If ever there was a time to make players very wary of simple terrain hazards, a well designed Mind Eye lair will do the deed.

Mind Eye's can see, after a fashion, using psychic clairvoyance, so they are as difficult to sneak up on as any other Beholder, and with a lair full of traps, bladed objects and slippery piles of coins, as well as the usual filth of a hundred slobbered and discarded meals.. Just trying to sneak across it's lair is no easy task. While it is true that they have nothing but stumpy growths where normally there are eye stalks, they do have a filmy white, nearly blind central eye, but most of the time they keep it closed, only opening it during combat, or while issuing portents of doom and dire threats, as per usual.