Neptune was the Roman god of water and the sea in Roman mythology and religion.
In 2266, Anton Karidian performed the line from Macbeth, Act II, Scene II: "Will all great Neptune's ocean wash this blood clean from my hands" on Planet Q. (TOS: "The Conscience of the King")
Neptune was worshiped on planet 892-IV too, but the Children of the Son considered him a false god. Neptune Bath Salts were named after him. (TOS: "Bread and Circuses")