The information in this article is from a fan game or fan fiction and is therefore not canon.
King Quest II: Romancing the Stones
King Neptune is the king of the Mer-people. He is the king of the watery world. He is known by many names, Neptune is the one which he is best known to humans. He is the keeper of the Birth Gem.
A book in the library in Kolyma suggests that Triton is the ruler of the seas. Thus it may imply it is just another name for Neptune.
In KQ2 VGA, he is the king of the Merfolk who has lost his trident to the Sharkees but Graham infiltrated their base and returned it to the King.