The Beholder is WotC Product Identity. It (and several other monsters) are not allowed to be used without getting a specific license from WotC to do so. Hence, not in the SRD.
There are others, too ... the mind flayer, displacer beast, yuan-ti, carrion crawler, umber hulk ... there might be a few more but I don't remember them at the moment.
If you need quick and easy replacmenets, you might want to try Lion's Den Press's Iconic Bestiary: Classics of Fantasy. It largely provides alternatives for creatures to replace those left out of the SRD.