Click a word to see its Strong’s Concordance entry:
So God created the great sea creatures and every living creature that moves, with which the waters swarm, according to their kinds, and every winged bird according to its kind. And God saw that it was good.
“The great sea creatures” = Hebrew word is Tannin, generally understood to mean giant sea serpents not unlike Leviathan in Isaiah 27:1 or Rahab in Isaiah 51:9. The same word is used to describe the creature that Aaron’s staff turns into in Exodus 7. In Isaiah’s references these creatures are used as poetic allegory, both in reference to Egypt and its leaders. Leviathan shows up again in Job, but again as poetic allegory, to explain the might of God. As Rashi touches on below, traditionally the ancient Jews believed that there were three great beasts, one for the sea, one for the land, and one for the sky – Leviathan, Behemoth, and Zizz. The mates were slain and preserved to be consumed in the afterlife (See Rambam’s commentary below).
However, there is no Biblical reason to presume these creatures actually existed, or that these words are not being used to describe another great beast of the sea. If, however, we go with a literalistic interpretation that holds that these are modern existing sea beasts (I have heard arguments for it being a giant squid) we must then account for it in the story of Aaron’s staff. Alternatively, I have heard it argued that this describes some of the many giant sea reptiles that existed in the dinosaur era. They would have been long extinct by Moses’ time, and no doubt would have created quite a stir when Aaron’s staff turned into one. I don’t have a way to rebut these ideas except to say that the best interpretation, and one which fits in with our interpretation/commentary on verses 2 and 6, is that this shows God’s dominion over all animals, even the mightiest beasts in the ancient imagination, and is intended to communicate a theological message rather than a historical one.
It is worth noting further that many pagan myths in the region believed the original deities to have been great sea serpents of a similar kind. The Egyptian creator deity Atum, for example, was believed to have originally been such a serpent who emerged from preexisting watery chaos. In a similar way, the original deities in the Babylonian myth, Apsu and Tiamat, were the preexisting waters themselves from which the other deities emerged. This contrasts starkly against the depiction of God as the Creator of the elements and all of the beasts that dwell within, further supporting the points made in verses 2 and 6.
If this word necessarily refers to such a creature, the transformation of Aaron’s staff must have been a shocking incident indeed, if it took the form, not of a snake, but of the original form of the Egyptian creator deity (a manifestation which would be within the power of God to create without that deity or the animal having ever actually existed)! The message to the Egyptian people sent by the manifestation and subjugation of the original form of their creator deity by the hand of the assistant of the prophet of the God who opposed them (not even by that God himself, but by one of his lackeys) was probably a very loud one.
Targum Onkelos: And the Lord created the great taninia and every living animal which moveth, which the waters generated according to their kind, and every fowl which flieth according to his kind; and the Lord saw that it was good.
Rashi: the…sea monsters: The great fish in the sea, and in the words of the Aggadah (B.B. 74b), this refers to the Leviathan and its mate, for He created them male and female, and He slew the female and salted her away for the righteous in the future, for if they would propagate, the world could not exist because of them. הַתַּנִינִם is written. [I.e., the final “yud,” which denotes the plural, is missing, hence the implication that the Leviathan did not remain two, but that its number was reduced to one.]- [from Gen. Rabbah 7:4, Midrash Chaseroth V’Yetheroth, Batei Midrashoth, vol 2, p. 225].
living creature: a creature in which there is life.
Rambam: On the first day, God created from nothingness Leviathan and his mate, killing the latter lest the two overwhelm the earth and salting away her flesh for the righteous in the world-to-come and, on the fifth day, God set Leviathan into its proper place in creation. God also generated all the beings which have souls and which move continuously, which the waters had generated by their continous movement, each according to its kind, together with each bird according to its kind. God confirmed them in their existence in God’s will.
Ibn Ezra: God created the great sea-fish; all living beings which slither, which the waters had swarmed forth, each according to its kind; and all winged-birds, according to their kinds. God understood that it was good.
Fun Fact: The primary legendary Pokemon of generation three were based on the three elemental beasts of ancient near eastern mythology. Kyogre is Leviathan, Groudon is Behemoth, and Rayquaza is Zizz.