I've always played Beholders as having Telekinetic powers, not emanating from a single eye-stalk, but as a full-time Special Ability... having no limbs, this only makes sense, to me. But since a Beholder just wouldn't seem 'right' without ten eye-stalks, I changed the former Telekinesis stalk to be the Random Stalk, and had a table something like this, making an already terrifying Monster even scarier, with the addition of an unknown ability:
1. Magic Missiles (one per 2HD?) 6. 'Burning Eye' (Burning Hands)
2. Cone o' Cold 7. Lightning Bolt
3. Blinding Flash 8. Ray of Enfeeblement
4. Feeblemind 9. Paralysis
5. Withering/Aging 10. Polymorph
So, I'm thinking Beholders start out as Groeningesque single-stalked 'tadpoles', and a new stalk is randomly rolled for each additional HD, from the classic Monster Manual description. To introduce the 'Random Stalk' element, there is a 1 in 10 chance, cumulative, that the newest stalk is instead the Random (10% at 1HD, 20% at 2HD, etc).
Might also be fun if they also advance in Intelligence as well, only attaining 'Exceptional' Intelligence - or greater(?) - at higher HD. Imagine encountering a 1HD Beholder on the first level of the dungeon! It says it has a Death Ray...