Albert Barnes 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)

Fiery serpents - The epithet (Deuteronomy 8:15; Isaiah 14:29; Isaiah 30:6) denotes the inflammatory effect of their bite. The peninsula of Sinai, and not least, the Arabah, abounds in mottled snakes of large size, marked with fiery red spots and wavy stripes, which belong to the most poisonous species, as the formation of the teeth clearly shows.