Let's talk about Beholders! They are probably the most iconic D&D monster around and trace their origin back to the early Gygax material. These bastards present a challenge for any group. Let's talk about why.
Beholders are extremely intelligent and arrogant creatures. They are classified as aberrations and posses magic that can bend reality. In fact, sometimes when they sleep and dream of other Beholders it actually creates a new Beholder. Of course they then are likely to fight each other until only one is left, but sometimes this creates new colonies. On of the very few silver linings is they are solitary creatures more often than not.
The large eye of a Beholder extends a cone of anti-magic in the direction it is looking. The Beholder also has 10 additional ray attacks at its disposal - an answer for every weakness. You get hit with a charm ray and you cannot attack the Beholder for hours. These rays are being cast at the spell equivalent of 13th level but they never run out! It's brutal.