What animal is a aquarius?

Answer 1

A human. Specifically, in mythology the constellation represents Ganymede, cup-bearer to Jupiter.

Perhaps you're thinking of Aquila, the eagle, a different constellation.

In addition to the similarity in names, there is a mythological relationship between the two constellations: when Jupiter transformed into a bird to carry Ganymede off, he transformed specifically into an eagle. There's also a spacial relationship between the two constellations; Aquila is right next to Aquarius.

Answer 2

It isn't. Most of the constellations of the Zodiac, and the corresponding astrological signs, are related to animals; a few are not. "Aquarius", which means "water-carrier", is simply a person carrying water.