Callipygian adjective : having shapely buttocks
While just how common callipygian folk are is in the eye of the beholder, your humble dictionary ventures here to suggest that callipygian folk are common enough to justify way more use of this word than we currently see. The less common variant callipygous is also a reasonable option. English got both from the Greeks of long ago, who clearly knew how to appreciate shapely buttocks. The Greek kallipygos has its origins in kallos, meaning "beauty," and pygē, meaning "buttocks."