Charles Ellicott Commentary Numbers 21:6

"And Jehovah sent fiery serpents among the people, and they bit the people; and much people of Israel died." — Numbers 21:6 (ASV)

And the Lord sent fiery serpents ... — Hebrew, the serpents, the seraphim (i.e., the burning ones). (Isaiah 14:29; Isaiah 30:6.) The word appears to denote a particular kind of serpent, as in the following verse. Some think that they were so called because of the bright fiery red upon their heads; others because of the blazing sunbeams on their scales; and others because of their inflammatory and poisonous bite. Venomous snakes are said to still abound in the Arabah.