Albert Barnes Commentary Numbers 31:8

"And they slew the kings of Midian with the rest of their slain: Evi, and Rekem, and Zur, and Hur, and Reba, the five kings of Midian: Balaam also the son of Beor they slew with the sword." — Numbers 31:8 (ASV)

And they slew ... were slain ... - Render: And the kings of Midian they put to death, besides those who fell in the battle; namely, etc. From this, it would seem that besides these five, put to death after the battle, there were other Midianite kings who perished fighting. The five chieftains here mentioned were vassals of Sihon the Amorite (Joshua 13:21).