The hippocampus is supposed to be pulling poseidon's chariot, and poseidon is the god of the sea, in the tripartite division of existence into the
heavens, earth, and ocean, with jupiter, pluto, and neptune ruling over them respectively.
The thing is, there are some suggestions that poseidon wasn't orignally a sea-only god. His worship is associated with the horse, with their
thundering hooves, who causes earthquakes and rules with the trident. Its possible that poseidon simply represents the primary god of one group of
people, who then became over-come, whether by force or anything else, by another people that primarily worshiped zeus. That would explain poseidon, a
water god, being associated with things like horses, exclusively land animals.
Also, its possible that the greeks and romans dreamt up the idea of the 'hippocampus' upon seeing regular sea horses.

