Yeah ok, so it's not surprising that the god of chariots would also be the god of horses...
...doesn't really explain why the sea god has domain over these things. maybe it was through synthesis of neptune with another god. all i know is that in the old vedic system of hinduism, horses were the domain of indra, who was the equivalent of jupiter.
or it just is and asking why is stupid. probably this.
EDIT: http://en.wikipedia.org/wiki/Neptune...unus_equestris Apparently Poseidon was associated with horses before the association with the sea. It looks like the association is explained through the god's violent nature as the "earth-breaker" as well as the association of horses with underground springs. And it might be by the influence of the Greeks that Neptune was likewise made to be associated with horses, by synthesizing some aspect of Consus with Neptune under the epithet "Neptune Equestrus."
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