Mind flayers are basically mini Cthulhu.
There is one aberration though that, while pigeonholed as monster, thrives on those grounds. They are the floating bags of eyeballs and fun that are beholders. As one of the Dungeons & Dragons classic monsters (like mind flayers), beholders have unfortunately not crossed over into other games like Pathfinder and Lamentations of the Flame Princess where they are sorely missed. Luckily, the conversions aren't that hard, so there are fan made "eye beasts" that terrorize adventurers, but since they're not printed in a book, it's not the same. There's not full color glossy art next to a stat block in a pretty hardback outside of what Wizards of the Coast puts out.
Awesome art like this.
That also means that it's nearly impossible to find a good miniature for them. Reaper makes an "eye fiend" mini, but it has tentacle legs and doesn't float so it isn't the same. Wizard's D&D minis line has a few, but like much of that line, they're either hard to find, expensive, corny, or all three. Plus those plastic minis have crappy bases and don't stand up well. Que sera. I don't use minis much anyways, but I like having them on the occasions that I do. Plus they're pretty on my shelves.
And the one from Reaper is pretty cool...
The great draw of beholders is their eye rays. Sure, there's a pre-set list of what eyes do what, but that's beyond easy to switch out, and it's awesome to see a player's face when an eye does something that isn't in the book. They also completely screw over parties. Back the beholder into a corner, turn the central eye and its antimagic field on the spellcasters, and let the eye rays or high damage mouth deal with the fighters and rogues and rangers. They can't be flanked because of the eyes, meaning rogues do crap against them anyways, and fighters fail the necessary will saves from eye beams with alacrity. Paladins are one of the few threats, and an antimagic field and some munching from the outset do pretty well for them.
They even have ecology and different breeds if a GM feels the need to include all that. The AD&D Monstrous Manual details much of it and most of the breeds, making it a good read. Dealing with beholder culture means quite high level characters though, so it's not something I've found much use for. Still, as bosses of a dungeon, cool enemies controlling kobold tribes, or a wandering beasty on a foggy, ruin covered moor, beholders are freaking awesome.
This party is probably screwed.