There is a select list of "original" D&D monsters. That list includes the Purple Worm, Umber Hulk, Displacer Beast, Rust Monster, Owl Bear, Blink Dog, Carrion Crawler and Stirge. If what I have read on other blogs and boards is correct, many of those original D&D monsters were inspired by miniature monster figures, that one of the early D&D players bought and supplied to Arneson's Blackmoor campaign.
I suspect that the Beholder (above, from the WOTC Deathknell set) was not one of those figures, and was likely the creation of Gygax. The Beholder first appears on the cover of Greyhawk (OD&D Supplement I), and receives one paragraph of description, and a table listing the functions of each of the Beholder's eleven eyes.
The beholder began as a 3-foot diameter monsterous globe with a central eye, mouth and 10 eye-stalks. Each of the eyes had different powers, for example, anti-magic ray, charm person, disintegrate, telekinesis, and so on. Over the years, the Beholder has been re-envisioned (at least in size), with the Beholder miniature shown above being roughly 10 feet tall, not including the added height of the eye-stalks.
This is one of the older WOTC versions of the Beholder. The most recently WOTC-produced Beholder figure is easily twice this size.