Genesis 36:39

And Baalhanan, the son of Achbor, died, and Hadar reigned in his stead; and the name of his city was Pau; and his wife's name was Mehetabel, the daughter of Matred, the daughter of Mezahab. Since the relatives of Hadar are noted so carefully and his death is not mentioned, he seems to have been king at the time of Israel's sojourn in the wilderness. It was Hadar, then, who refused the children of Israel passage through his land, Numbers 20:14. It appears from the list of kings that the monarchy in Edom was limited and that it was not hereditary, but elective, the tribal princes very likely choosing a king out of their midst whenever a vacancy occurred.