So this is how my old joke about beauty being in the eye of the beholder gets co-opted. Also, weren't beholders removed from Wizards' OGL?
Beholders weren't OGL content, but were published in editions where OGL content appeared. (Those would be the impure later additions our fine hostess won't have any truck with, but not editions so late that I won't have truck with them. I only play prime-numbered editions.).
Basically, if you go look up what the Linux-licensed, you won't find Beholders, but if you fork over money for 3rd edition hard-copy, Beholders (and Slaad, and Mind Flayers, etc.) are there plain as day.
I think it has something to do with TSR/WotC/Hasbro not having the full rights to that content.