King Neptune (born 5,000 years ago ), is the series' depiction of Neptune, the Roman God of the sea. In the universe of SpongeBob SquarePants, Neptune's the god and supreme ruler of the sea which makes him enemies with The Flying Dutchman. His name is often used in place of God's, with phrases such as "Dear Neptune" and "Neptune preserve her!". However, he has never explicitly been shown to be worshiped by the creatures of the sea. King Neptune first appeared in the episode "Neptune's Spatula", while a completely different looking Neptune appeared in The SpongeBob SquarePants Movie. Then the same Neptune from Neptune's Spatula appeared in Clash of The Triton. Neptune is voiced by John O'Hurley in the TV series, and by Jeffrey Tambor in the movie. His Greek counterpart is Poseidon.
King Neptune first appeared in the season one episode "Neptune's Spatula", where he appears after SpongeBob pulls the Golden Spatula from the ancient grease, making him Neptune's royal fry cook. However, Neptune refuses to accept such a lowly creature as SpongeBob as the chosen one, and challenges him to a cook-off where, if SpongeBob wins, he will become a god in Atlantis, but if he loses, he must give up fry cooking forever. The challenge is held at the "Poseidome", where the first one to make 1,000 patties wins. Using his godlike powers, Neptune wins the challenge just as SpongeBob finishes his first patty. To celebrate his victory, he gives the patties to the audience, who hate it. Neptune then tastes SpongeBob's patty, and loves it. He declares SpongeBob the winner and makes him a god. However, when SpongeBob learns that his friends can't join him in Atlantis, he does not want to go, and instead makes Neptune his "trainee" at the Krusty Krab. In this episode, Neptune is green, and has fairly realistic human features, has hair, a mustache, a beard, thick fingers, is very muscular, and has a different crown. This version of Neptune also appears in Battle for Bikini Bottom, where he summons SpongeBob and Patrick to the Poseidome to defeat a robot version of Sandy.
Neptune plays a central part in The SpongeBob SquarePants Movie. The movie also introduces Neptune's daughter, Princess Mindy. This version of Neptune is green-skinned and far more cartoony than the version in "Neptune's Spatula". He is also explicitly portrayed as a king rather than a god, and lives in a palace just outside of Bikini Bottom rather than Atlantis. This Neptune is very brutish and tyrannical, which he tells his daughter is necessary for him to be respected as a ruler. His crown is very important to him, not only because it is a symbol of his power, but because it covers his "thinning" (bald) head. When the crown is stolen by Plankton Neptune attempts to execute Mr. Krabs, who is framed for the crime. In SpongeHenge, Shubie denies Neptune's existence when an anchovy tells her that Neptune must be mad, saying, "Everyone knows poseidon is the ruler of the undersea."
A picture of King Neptune, as seen in "Neptune's Spatula", is shown in "Krusty Towers". Squidward refers to this Neptune as "Neptune XIV", suggesting that there are multiple King Neptunes, and possibly explaining his inconsistent appearance.
Judging by his different appearances, it is reasonable to conclude that Neptune is actually two different characters, with the first one referring to the Roman deity of the same name, and the second one works as the actual King of SpongeBob's underwater world. Their different places of living, the fact that he doesn't know SpongeBob, and Princess Mindy only being in the movie reinforces this theory.
There is also the fact that in Krusty Towers Squidward says "King Neptune XIV would be nice." Which claims that the three versions of Neptune through out the series are not the same character and instead could just be different monarchs of the royal family and are probably related.
Another possible explanation is that he has gotten older from the show to the movie resulting in his loss of hair, because Stephen Hillenburg did establish that the movie does take place after the series. However this is hard to say seeing as The movie takes place 36 years after the pilot episode (because of Spongebob's 374 straight employee of the month awards), and so far Neptune (from Neptune's Spatula) has been relieved to have been around for 5000 years, meaning that 36 years (or less as his 5000th birthday took place after Spongebob got his job) probably won't age him that much. Plus there is the fact that he is a god (hence immortality) so he might not age at all once he reached a certain age point.
The SpongeBob SquarePants Movie (video game)- King Neptune's role is the same as in the Movie: his crown goes missing and he accuses Mr. Krabs for taking it. He later sends SpongeBob and Patrick to find it, allowing Plankton to take Bikini Bottom for himself. His crown is returned and he frees Mr. Krabs. Under the control of Plankton, he is the final boss of the game.
SpongeBob's Atlantis SquarePantis - King Neptune makes a cameo in a painting in the Bikini Bottom Museum as an object SpongeBob must photograph. The painting is also unlockable in the gift shop with The Sirens added in it.
The Clash of Triton (PC Game)- Triton, King Neptune's son, captures King Neptune on his birthday at the Krusty Krab. After his and Queen Amphitrite's tridents fall near SpongeBob and Patrick they gain powers and defeat Tritons underlings (The Monsters from the Party). They defeat Triton, restore Bikini Bottom to it's former glory, and reunite Triton with his proud father.