The theology of Neptune is limited by his close identification with the Greek god Poseidon, one of many members of the Greek pantheon whose theology was later tied to a Roman deity.The lectisternium of 399 BC indicated that the Greek figures of Poseidon, Artemis, and Heracles had been introduced and worshipped in Rome as Neptune, Diana, and Hercules. It has been speculated that Neptune has been conflated with a Proto-Indo-European freshwater deity; since the Indo-Europeans lived inland and had little direct knowledge of the sea, the Romans may have reused the theology of a previous freshwater god in their worship of Neptune. Servius explicitly names Neptune as the god of rivers, springs, and waters;he may parallel the Irish god Nechtan, master of rivers and wells.[citation needed] This is in contrast to Poseidon, who was primarily a god of the sea

Neptune has been associated with a number of other Roman deities. By the first century BC, he had supplanted Portunus as the god of naval victories; Sextus Pompeius called himself the "son of Neptune". For a time, Neptune was paired in his dominion of the sea with Salacia, the goddess of saltwater. Neptune was considered the legendary progenitor god of the Falisci (who called themselves Neptunia proles), joining Mars, Janus, Saturn, and Jupiter as the deific father of a Latin tribe