Dreadglare the Spellsire is always available.
Beholder mages give up their central eye though, and their eye stalks become "spell-stalks" instead as they gain levels, with each capable of casting one spell of a particular level each round as a free action. So each round, a 10th-level beholder mage can cast up up to 10 spells using its "spell-stalks" as a free action, as long as they are all of a different level, 0th-level through 9th-level.
Their casting is Intelligence-based, they learn spells like wizards, but know and cast them on the fly like sorcerers.
They are pretty nasty.