Albert Barnes Commentary Exodus 7:9

"When Pharaoh shall speak unto you, saying, Show a wonder for you; then thou shalt say unto Aaron, Take thy rod, and cast it down before Pharaoh, that it become a serpent." — Exodus 7:9 (ASV)

Your rod - Apparently the rod previously described (Exodus 4:2), which Moses on this occasion gives to Aaron as his representative.

A serpent - A word different from that in (Exodus 4:3). Here a more general term, תנין tannı̂yn — is employed, which in other passages includes all sea or river monsters and is more specially applied to the crocodile as a symbol of Egypt. It occurs in the Egyptian ritual, nearly in the same form, “Tanem,” as a synonym of the monster serpent which represents the principle of antagonism to light and life.