Crystalloid monsters were believed to be an alien race from somewhere in outer space that lived billions of years before humans ever even existed, likely during the geological period known as the Hadean.
They had the appearance of a crystal serpent with a geometrically shaped head, torso, pelvis, and a tail; but no arms or legs of any kind, at least none that was visible. They simply moved about by creeping on the ground similar to a snake.
They also laid eggs to reproduce. Their eggs were black and impervious to drills capable of even cutting through diamond, meaning the shell of the egg was even harder than diamond. It should be noted however, that a double-dose of heat vision from Superboy and Krypto was able to penetrate the egg shell though.
Crystalloid mothers were very nurturing toward their young, and although they were generally not a hostile race, they were known to use violence when their young were threatened, which is why they were called "monsters" by the people of Earth.