This answer states that "illithid" is Undercommon for mind flayer. Is there a similar word for beholder?
Beholders are powerful creatures and undercommon is one of their languages. If there wasn't a word for them, then they will have made one (or several which they all believe is the best word for their kind).
There doesn't appear to any WOTC-official undercommon word for them. The best collection of undercommon translations appears to be this one and it includes no words for Beholders or for looking at things, eyes, or really anything ocular.