Neptune, also known as Poseidon, was the Greco-Roman god of the sea and horses.
Neptune was one of the children of Saturn and Ops, alongside Jupiter, Pluto, Juno, Ceres and Vesta. All save for Jupiter were swallowed by their father, he forced Saturn to vomid the five other gods out and united they overthrew him.
Jupiter, Neptune and Pluto then drew straws on which aspect of the universe they would rule: Jupiter became the lord of the skies, Neptune lord of the seas and Pluto became lord of the Underworld. As ruler of the seas married the goddess Salacia, and had a son named Triton, but much like his Jupiter, he had many affairs with mortal women, resulting in the heroes (such as Bellerophon) and giants.
Neptune would also get into a feud with his niece Minerva over the city of Athens, as both deities wanted to be the patron of the city. Minerva won the favour of the people, who built a great temple for (the Parthenon). For revenge Neptune raped one of her priestesses, Medusa, within her temple, tarnishing it. Minerva punished Medusa for this, turning her into the gorgon-like monster that would eventually be slain by the hero Perseus.
He was one of the many pagan gods demonised by Christianity, and we has notably mentioned in a sermon by Saint Eligius, in which he tells that Christians should associate with pagan gods or practices. His primary weapon, the trident, would likely inspire the popular image of Satan carrying a pitchfork.