This answer states that "illithid" is Undercommon for mind flayer. Is there a similar word for beholder?
Beholders are powerful creatures that are typically found in the underdark. This means that they will certainly deal with other creatures from the underdark, as evidenced by the fact that one of their languages is Undercommon, and those creatures will definitely have some way of referring to them other than "those terrifying egomaniacal abominations with lots of eyes".
I originally said that due to their god-complex, they would create their own word for themselves if there wasn't one already, but as Carcer has pointed out in the comments, apparently 3.5's "Lords of Madness" took this insanity one step further. Being associated with other beholders in any way would be so mortifying for a beholder that the Beholder language itself has no word for "Beholder" and all words for them as a species are invented by other races as a result.
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.
\$\begingroup\$ According to the 3.5e Lords of Madness, the Beholder language actually has no word to describe their own species and they would generally find the idea of being associated with other beholders in any way mortifying - so any names used to refer to the species are always thought up by other races, not beholders themselves. \$\endgroup\$– CarcerJul 13, 2020 at 23:42
1\$\begingroup\$ I really like that little nugget of information, and although I'm sure other creatures speaking Undercommon must have made a word for them I'll edit my answer to mention it. \$\endgroup\$ Jul 13, 2020 at 23:50
1\$\begingroup\$ Also, I may use this in my games from now on. If my players encounter a beholder and call it a beholder, I'll have them get insulted and angry and start telling them all about how and why they're unique and special. \$\endgroup\$ Jul 14, 2020 at 0:04