Planet of origin: Godag
Chasch are an old bidedal reptilian species that traveled the stats through slower than light means and settled on several worlds. Through time, they have evolved in several subspecies. The ones that look closer to the original are the old chasch. The only recorded contact mankind had with the CHasch are on the planet of Tschai, where old chasch, Blue Chasch and Green Chasch coexist but there might be other Chasch subspecies on other worlds.
The Old CHasch arrived on Tschai about a hundred thousand years ago. They are ivory colored with silver-plated eyes that can move independently from one another. They are the smaller of the lot and their aspect is somewhat grotesque.
Ten thousand years later, a second wave followed: the Blue Chasch, coming from a planet colonized in remote times by common ancestors, arrived on Tschai and started a conflict with the Old Chasch for the dominion of the planet. Both groups hired Green Chasch as mercenaries. As the conflict resolved itself in a stalemate, the Green Chasch also settled on the planet where they formed nomadic hordes in western steppes.
The Blue Chasch look much stronger, they are covered with bluish pointed scales. They have have chitinous armoured shoulders that curve backward to form a dorsal shell, and their skull is crowned with a bony edge. Their nasal apparatus is quite complex and they can distinguish people and emotion through their scent.
The Green Chasch look much like green version of the Blue ones but much larger, as they tower at about
Something common to all the Chasch, in addition to hatred which they feel for the other varieties of their race, is their very particular sense of humor, a sense of humor that can often be fatal for anyone they capture.
Like the rest of the alien species who have settled on Tschai, they have selected to the humans during thousands of years that physically and psychologically resemble them to serve as their servants, the Chaschmen.
Source: Planet of Adventure (Jack Vance)
Note: Other reptilian species bearing ressemblance to the Chasch include the Unas.