And they slew ... were slain ... - Render: And the kings of Midian they put to death, beside those that fell in the battle; namely, etc. From which it would seem that beside 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.
Related Commentaries of Numbers 31:8
Numbers 31:8
8 And they slew the kings of Midian, beside the rest of them that were slain; namely, Evi, and Rekem, and Zur, and Hur, and Reba, five kings of Midian: Balaam also the son of Beor they slew with the sword.