He is the god of water and the sea in Roman mythology. He is analogous with, but not identical to Poseidon the Greek god.
Neptune was one of the three brothers presiding over the three realms of the universe, Heaven, Earth and the Netherworld. Jupiter and Pluto are the other two brothers.
Neptune was associated as well with fresh water. For Latins, who were not a seafaring people, the primary identification of Neptune was with freshwater springs.
Like Poseidon, Neptune was worshiped by the Romans also as a god of horses, under the name Neptunus Equester, a patron of horse-racing.
The theology of Neptune may only be reconstructed to some extent as since very early times he was identified with Poseidon the Greek god.
Such an identification may well be grounded in the strict relationship between the Latin and Greek theologies of the two deities.
It has been argued that Indo-European people, having no direct knowledge of the sea as they originated from inland areas, reused the theology of a deity originally wielding power over inland freshwaters as the god of the sea
This feature has been preserved particularly well in the case of Neptune who was definitely a god of springs, lakes and rivers before becoming also a god of the sea.
This is testified by the numerous findings of inscriptions mentioning him in the proximity of such locations.
Servius the grammarian also explicitly states Neptune is in charge of all the rivers, springs and waters.
He may find a parallel in Nechtan – the Irish god, master of the well from which all the rivers of the world flow out and flow back to.
Poseidon on the other hand underwent the process of becoming the main god of the sea at a much earlier time.