Albert Barnes Commentary Exodus 4:3

"And he said, Cast in on the ground. And he cast it on the ground, and it became a serpent; and Moses fled from before it." — Exodus 4:3 (ASV)

A serpent - This miracle had a meaning which Moses could not mistake. The serpent was probably the basilisk or Uraeus, the Cobra. This was the symbol of royal and divine power on the diadem of every Pharaoh. The conversion of the rod was not merely a portent; it was a sign, at once a pledge and representation of victory over the king and gods of Egypt!