Poseidon

Neptune has hair, a mustache, a beard, chest hair, thick fingers, and a different crown. In the SpongeBob movie, Neptune is "Slim" and has a trident. As previously suggested, it's possible these are, in fact, different individuals, as there may have been several King Neptunes who succeeded the original series, or possibly he grew older due to the fact that the movie takes place after the series.

King Neptune is the king of the seas. He is the husband of Queen Amphitrite. He is arguably the most powerful being in the sea (along with Poseidon). He is an evil, fiery-tempered merman. He likes to shoot people with his trident, but Mindy Neptune (daughter of his kind) often convinces him to have mercy. He is very defensive about his hair; he says he isn't bald, just thinning. He also covered his head with a paper bag, as seen in the image at the bottom right of the page. He was even overly concerned about his bald head with this kind of "Plankton Conquers Bikini Bottom" warning. He used to be very young, with a full head of hair. When he's out of the water, like in "Party Pooper Pants," his hair is lighter. When he comes back out, some of his aging continues. As seen in "The SpongeBob SquarePants Movie," he's bald, or "thinning," as he puts it, though obviously bald. In "Celtic Sponge," it's revealed that he's sometimes called Poseidon, the Greek name for Neptune. King Neptune has changed considerably since his first appearance in the episode "Neptune's Spatula," where he was young, muscular, and had long hair. In that episode, he claimed to be the king of Atlantis, but in "Adventure in Atlantis," His Royal Highness, or LRH for short, is the king.