Sea King is a generic term used by sailors, pirates and civilians to refer to any number of giant reptiles, amphibians, or mammals that inhabt the Blue Seas and Grand Line. The creatures featured in One Piece are modeled after medieval tales of mythical sea monsters responsible for many disasters at sea, most of which were due to piracy or storms. Sea Kings in One Piece are a rarity in the Blue Seas, though they are a bit more common in Grand Line; the largest specimens of various Sea King species make their home in the Calm Belts that encircle the globe, which serve as breeding grounds for these monsters. It is believed that the presence of the Sea Kings in the Calm Belts are the cause of the doldrums that occur there.
Dispositions toward Sea Kings are not uniform across the oceans; in the Blue Seas, their rarity is a source of wonder and terror, for they are considered more like legends than actual living beings. In Grand Line, Sea Kings are seen as more like pests, destroying valuable cargo ships and inhibiting commerce between nearby islands. In spite of this status, killing a Sea King is considered a feat of great strength or fighting prowess among many cultures, especially including the Viking Warrior giants of Elbaf.
Numerous Sea Kings populate the One Piece world; the following is a list of those who have already made appearences.
Lord of the Coast: An eel-like carnivorous Sea King of large size, first introduced in Chapter 1 of One Piece. Although his territory presumably includes all of East Blue, Lord of the Coast has a particular affinity for the island Windmill Village is built on. He is the first Sea King introduced in One Piece, where he devours Higuma the Bear and bites off Shanks' arm before fleeing. Ten years after his carnage, Luffy takes revenge for Shanks' lost arm but knocking out Lord of the Coast with a single Rubber Rubber Pistol.
In the first One Piece prototype Romance Dawn, Lord of the Coast was originally a large shark closely resembling the famous Great-White Shark Jaws. Although a young Luffy was abducted by Higuma in the final version of One Piece, in the original Luffy was merely galavanting off at sea when he was attacked; this shark devoured half of Luffy's rowboat before biting Shanks' arm and fleeing.
Moo Moo: A large Sea King originally native to Grand Line, this particular specimen is a full-fledged member of the Arlong Pirate Gang. Moo Moo belongs to a peculiar species of Sea King that has no concerete origin in any real of fictional animal; his body is simmilar in appearence to those of manatees or dugongs, but he also possesses scales. Additionally, Moo Moo possesses several bovine features, notably horns, a snout, and a nose ring; however, although modeled after a herbivore he is in fact carnivorous and possesses a full set of sharp teeth. In stark contrast to every other Sea King in the series, Moo Moo is intelligent and can understand human (and presumably animal) speech, though he is also immense strong and is a powerful swimmer.