Typhon was a very powerful adversary to the gods. One bad day, Typhon decided to come up Mount Olympus. In order to escape him, most of the gods turned themselves into animals, to gain speed and agility. Only Aphrodite and Eros, the gods of love, remained. At the last moment, two fish appeared and Aphrodite and Eros jumped on their backs and escaped. As a means of honouring their service, the two fish were placed in the heavens, as the constellation Pisces. This is the month of Purim and the Great Dionysia.
See source texts: https://www.theoi.com/Ther/Ikhthyes.html