Neptune is the god of the sea among the Romans. He was not a very powerful god, and little is known of his origin. When he was first introduced in Rome, he already had all the characteristics of the Greek Poseidon. Despite the fact that his cult grew after his equation with Poseidon, Neptune was far less popular among sailors than Poseidon was among the Greek mariners.

Neptune was held in much higher regard as Neptune Equester, the god and patron of horse-racing and horses. One of temples was located near the Circus Flaminius, one of the larger trace-tracks. Another sanctuary was in the Campus Martius (25 BCE) were the Neptunalia was celebrated on July 23. The trident is Neptune's attribute.

Picture from this site.

http://www.pantheon.org/articles/n/neptune.html