And Moses and Aaron went in unto Pharaoh, and they did so as the LORD had commanded: and Aaron cast down his rod before Pharaoh, and before his servants, and it became a serpent. It became a serpent - תנין tannin. What kind of a serpent is here intended, learned men are not agreed. From the manner in which the original word is used in Psalms 74:13; Isaiah 27:1; Isaiah 51:9; Job 7:12; some very large creature, either aquatic or amphibious, is probably meant; some have thought that the crocodile, a well-known Egyptian animal, is here intended. In Exodus 4:3 it is said that this rod was changed into a serpent, but the original word there is נחש nachash, and here תנין tannin, the same word which we translate whale, Genesis 1:21.
As נחש nachash seems to be a term restricted to no one particular meaning, as has already been shown on Genesis 3; See Clarke's note on Genesis 3:1. So the words תנין tannin, תנינים tanninim, תנים tannim, and תנות tannoth, are used to signify different kinds of animals in the Scriptures. The word is supposed to signify the jackal in Job 30:29; Psalms 44:19; Isaiah 13:22; Isaiah 34:13; Isaiah 35:7; Isaiah 43:20; Jeremiah 9:11, etc., etc.; and also a dragon, serpent, or whale, Job 7:12; Psalms 91:13; Isaiah 27:1; Isaiah 51:9; Jeremiah 51:34; Ezekiel 29:3; Ezekiel 32:2; and is termed, in our translation, a sea-monster, Lamentations 4:3. As it was a rod or staff that was changed into the tannim in the cases mentioned here, it has been supposed that an ordinary serpent is what is intended by the word, because the size of both might be then pretty nearly equal: but as a miracle was wrought on the occasion, this circumstance is of no weight; it was as easy for God to change the rod into a crocodile, or any other creature, as to change it into an adder or common snake.