Pick your Parshanut Preference
1. As per Rashi it is a SNAKE
2. As per R Hirsch and others it's a CROCODILE.
Advantages to #1
A it matches the symbol in P. Sh'mot 4:3
B Rashi says so
Advantages to #2
A Sh'mot SAYS Nachash - here it's Tanin. We naturally expect a distinction!
B. Crocodiles were symbols of Egypt, thus more symbolic. As Haftarah Vo'eira [EZE 29:3] says TaniM that is HaRoveitz. Crocodiles crouch, snakes don't
C. If you read Taninim in P. Breiseet 1:21 as great lizards, this matches it a bit better
Pick your preference