In all likelihood they, like the Midianites, were desert dwellers. They could very well have been from the sons of the concubines that Abraham had with Keturah, among which was Midian. Abraham sent these “eastward to the east country” (Genesis 25:6).
3 For whenever the Israelites planted crops, the Midianites and the Amalekites and the people of the East would come up against them.