Here are the building instructions:
The flexible tube pieces come in Space and Town sets. If you do not have any, they are part number 4558850. You can get some by going to the Pick a Brick site and entering the part number in the 'Element ID' box.
Aside from the tubes and the translucent colored eyeballs, a couple Mixel sets will give you all the parts you need to make a Beholder similar to this one. The hinge pieces I used to make the jaw have been replaced by separate clips and rods in modern sets, and most Mixels have something like a big jaw that uses them. These would work just as well.
When you make your beholder, be creative and use the parts you have. It does not have to be exactly like this one. After all, it is part of D&D lore that no two beholders are the same.