Refer to 7:3:3. (Also refer to 4:17:2.1:1 and its notes.)

Rashi: "Tanin" is a snake (or a viper - The Living Torah). ¹

Seforno (to 8:8): "Tanin" is a crocodile. ²

Ibn Ezra: Aharon's staff and that of Moshe ¹ are one and the same. ²

Ha'amek Davar (citing other Midrashim) ³ : It was Aharon's staff that now turned into a Tanin, whereas Moshe's staff had turned into a Nachash (in 4:3). ⁴

Targum Yonasan: It was a prediction that the whole world was destined to hear the cries of the Egyptians when Hashem will break them, just as the whole world heard the screams of the snake (in Gan Eden) when Hashem cut off its hands and legs.

Da'as Zekenim: Because Pharaoh was vain, and therefore became known as 'the great Tanin;' and because, just as a snake twists and turns as it moves along, so too, did Pharaoh twist and turn. ¹

Rosh (to 6:3): Pharaoh is called a snake - "ha'Tanim ha'Gadol" (Yechezkel 29:3). 'You made yourself like a snake, and swallowed twelve Matos (tribes) - you will eventually expel them, and I will return you to Tohu va'Vohu (nothingness) - just as I turned the snake back into a staff.

Oznayim la'Torah: It was Moshe's staff that turned into a crocodile (a sea-serpent), which is the translation of 'Tanin' -- because Pharaoh bore the title 'ha'Tanin ha'Gadol." ²

Maharal: Refer to 7:9:6.

Maharal #1 (Gevuros Hashem beg. Ch. 32, p. 121): In front of Am Yisrael, Moshe was to perform all three signs, to help them reach full Emunah. ¹ Pharaoh, on the other hand, was not going to believe in any event! It was sufficient to perform one sign (for he had requested it). ²

Maharal #2 (ibid. Ch. 27, p. 110): The three signs represent three stages in Hashem's carrying out justice against man. ³ The first sign came to instill fear - and fear would be sufficient for Pharaoh at that point in time (for if the Egyptians were frightened by the Tanin, they ought to fear Hashem, who had created it!). Ultimately, the Egyptians would be taught the other two lessons the hard way. ⁴

Maharal #1 (Gevuros Hashem Ch. 27, p. 110): Refer to 7:9:5:2.

Maharal #2 (Gevuros Hashem beg. Ch. 32, p. 121, explaining the Midrash): The snake of Bereishis was cursed with ten curses; ¹ this would hint to Pharaoh that he would be hit with ten Makos.

Maharal #3 (ibid.): A Rasha is like a dog; he is despised by all (i.e. he is far removed from reality). One strikes a dog with a stick; a stick is used to force those who are unwanted to keep their distance. (Thus, the sign chosen to perform before Pharaoh, was the one that involved a Mateh.)

Maharal #4 (ibid.): (Evil) kingdoms kill with a whisper (i.e. they first imprison, and then easily get rid of their opponents without publicity); so too, a snake hisses and kills. Also, just as a snake winds a crooked path, so too may a king choose any path, demolishing whatever is in his way. ²

Maharal (ibid., p. 122): Afterwards, the snake turned back into a staff. This reflects what would happen to Pharaoh! Every time Moshe and Aharon left his presence, he would rant and rave, 'If that Ben Amram comes back, I will kill him, crucify him, burn him up!' Yet the moment Moshe entered, Pharaoh became mute as a stick.