Sea Beasts are a class of large aquatic mammals that inhabit the Grand Line, notable for their uncanny resemblance to surface mammals. The first Sea Beast encountered in the series is the Sea Cow Momoo.
Their appearance is generally based around mammalian surface animals, such as cats, cows, elephants and many other kinds of mammals. The main difference between them and their surface counterparts are the added scales along their bodies, their fish-like fins and tail fin instead of legs and/or tails, and the ability to breath underwater through presumably gills. The majority of sea beasts are gigantic in size, easily being around the size of a kraken (which can have an estimated length of 300m (984')) and even their young can be quite huge. However smaller sea beasts, such as the Sea Squirrel exist as well which are only a bit larger than humans.
While not as aggressively carnivorous as Sea Kings (which are, in fact, one of their natural predators), most Sea Beasts will indiscriminately attack smaller animals, including both humans and merfolk. Nevertheless, they are much easier for powerful individuals to intimidate and domesticate; around Fish-Man Island in particular, they are commonly captured and harnessed by experienced pirates for underwater transportation, with the New Fish-Man Pirates being known to own an entire pack.
Sea Cows (海牛 Kaigyū?) greatly resemble normal, land-based cows, with curved horns, fleshy noses, and "patchy" coloring—though they are still carnivorous, with appropriately sharp teeth. So far, the only known specimen is Momoo of the Arlong Pirates, who stands at a height of 36 meters (118'1" feet) and a weight of 16 tons (32,000 lbs).
They are classified as type "A" creatures "Big Friendly". This stands somewhat in contrast with their behavior of attacking and overturning ships, though it has been indicated that they do not actually eat human beings.
Sea Cows are the only Sea Beasts known to be capable of moving on land (as the Arlong Pirates had—implicitly—let Momoo rampage inland through Gosa Village). The fish-like scales on their bodies are actually their hardened skin, protecting their vital organs from external attacks as an armor. Besides helping them swim as fast as 70 km/h (43.5 mps), their fins contain retractable bones, sharp enough to act as claws.
Just like normal cows, sea cows have four stomachs; bacteria and parasites dwell inside them, digesting anything the cows choose to eat, including poison from venomous animals, making these creatures essentially immune to food poisoning.
Their name may be a pun on real-life dugongs and manatees, which are commonly referred to as "sea cows".
Sea Cats (海ネコ Umi Neko?, Cat Sharks in the 4Kids-dubbed anime), like ordinary cats, possess large, pointed ears, slit-pupiled eyes, distinct philtrum lips, and frontal limbs ending in paws—which they sometimes raise in a "beckoning" gesture, akin to a maneki-neko, on surfacing. While grown specimens are as large as any other Sea Beast, juveniles—the only juvenile Sea Beasts depicted thus far—are "only" slightly larger than the 3.01m (9'10") Jinbe.
They are classified as a type "A" creature, "Big Friendly". Despite their preying on ships, the Kingdom of Arabasta considers them religiously significant ("endangered" in the 4Kids dub), and strictly forbids them from being harmed. Notably, the Tomb of the Kings contains multiple statues carved in their likeness.
A Sea Boar (海イノシシ Umi Inoshishi?) appeared in Hatchan's cover story where Hatchan saved it from an attacking sea monster. It turns out that earlier the sea boar had eaten Camie and Pappag and spat them out shortly after being rescued.
The Sea Monkeys (シーモンキー Shī Monkī?) are sea creatures with monkey-like faces. Found after returning from Skypiea and before arriving at Long Ring Long Land island, they have powerful flippers that can cause tidal waves. The Fanged Toad Pirates' ship sank due to the waves they created.
They may be a reference to the "Sea-Monkey" shrimp marketed as novelty pets by many American toy companies (hence their species name being rendered purely in katakana—a convention used for no other Sea Beast).
In the manga, it is a giant Sea Beast, while in the anime, it is a shapeshifting animal. However, its transformations are limited to its original size, roughly that of a real raccoon, such as transforming into a doll-sized Nami. Lina is the only known person to own a sea raccoon. She and her pet, Nukky, appeared in the Spa island filler arc with her sister Sayo.
A Sea Rabbit (海兎 Umi Usagi?) is a sea creature resembling a lagomorph. One was discovered near the Red Line by Luffy, Brook, and Robin traveling in the Shark Submerge III, which consequently was defeated by a Gomu Gomu no Rifle. Upon defeat, it spits out Camie and Pappag. It's birthday is October 19th.
Sea Lions (海獅子 Umi Jishi?) possess faces and manes like those of normal lions (though, unlike Sea Cats, all four of their limbs end in flippers). By estimate, a typical adult is 55m (180' 5") long.
Thus far, the only specimen depicted was part of the New Fish-Man Pirates' army. which was later ridden by the Flying Pirates during their attack at Ryugu Kingdom. After the New Fish-Man Pirates were beaten, the sea lion and its fellow sea monsters went on to cause havoc by going on a rampage.
It may be based on the sea-lion, a mythical creature in heraldry with of the head and front limbs of a lion with the tail of a fish, or the merlion, another mythical creature with the head of a lion and the body of a fish. It is a visual pun on real-life "sea lions".
The only one seen is under the command of the New Fish-Man Pirates, and it appeared first alongside the Sea Lion and many other sea beasts under the crew's command. It was on a rampage, but stopped when some fish-men tried to frighten it with fire.
The kanji for sea bear can mean "fur seal" in Chinese, which the sea bear somewhat resembles.
The name may be a pun on the Japanese word for dolphin (海豚 Iruka?), the kanji of which literally translates to "sea pig".
Sea Dog (海イヌ Umi Inu?) is a Sea Beast resembling a dog. In Solo Journey of Jinbe, Knight of the Sea, one is seen working as a police officer, wearing a police hat and badge, and even having a mustache.
Though not (yet) formally named, several other Sea Beasts were depicted among the New Fish-Man Pirates' forces, including:
- A Sea Giraffe, which served as Hammond's—and later Hody Jones'—primary steed.
- A Sea Elephant, which served as Dosun's primary steed.
- A Sea Rhinoceros, which served as Zeo's primary steed.
- A Sea Gorilla, which served as Hyouzou's primary steed.
- A Sea Squirrel, which Hammond—unsuccessfully—tried to send against Luffy.
- A Sea Leopard.
After the New Fish-Man Pirates fell, most if not all of these joined the Sea Lion and the Sea Bear in an underwater town some distance from Fish-Man Island.
- A Sea Lamb
- A Sea Koala
- A Sea Horse, strongly resembling a "King" Yagara Bull.
It is unknown whether these Sea Beasts were also part of the New Fish-Man Pirates, or if they simply flocked together out of instinct.
Translation and Dub Issues
The Viz manga initially translated Momoo's class as the more generic "sea monster", with the term "Sea Beast" not being used until the translation of Volume 17's SBS (which confirmed that Momoo and creatures like him belonged to a category distinct from Sea Kings). Interestingly, the original Japanese term, Kaijū (海獣?), is used in real life for ordinary marine mammals such as dolphins and whales; it remains unknown whether those of One Piece (e.g. Laboon) are also categorized as such.