Such there were many in this wilderness, Deuteronomy 8:15, which having been hitherto restrained by God, are now let loose and sent among them. They are called fiery from their effects, because their poison caused an intolerable heat, and burning, and thirst in the bodies of the Israelites, which was aggravated with this circumstance of the place, that here was no water, Numbers 21:5.
Related Commentaries of Numbers 21:6
Numbers 21:6
6 And the LORD sent fiery serpents among the people, and they bit the people; and much people of Israel died.