The symbols for the planets, Moon, and Sun (along with the symbols for the zodiac constellations) were developed for use in both astronomy and astrology. The symbol for Neptune is the trident (a long, three-pronged fork or weapon) of Neptune, god of the sea.
The ancient Babylonians named the five planets (or "wandering stars") that were visible to the naked eye after their gods. The ancient Greeks followed suit, and later the ancient Romans translated these names into those of the Roman deities who corresponded to the Greek gods. These are the names we know today. The outer three planets were named in modern times, but according to the same tradition and manner as the ancient names.
Neptune was discovered in 1845 by Johann Galle and Heinrich D'Arrest in Berlin. The name Neptune had previously been considered for Uranus and was then proposed for the new planet. From the Roman mythological point of view, Neptune (or Poseidon in the Greek) was the brother of Jupiter (or Zeus) and son of Saturn (or Cronos). Moreover, the planet Neptune appears to be bluish-green, the color of the sea.
Image Credit: Lunar and Planetary Institute
Credit: Lunar and Planetary Institute