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.
Most sea beasts seen are dangerous and violent creatures that lash out with incredible ferocity. Despite this, even the largest and most ferocious of them can be tamed by powerful individuals, such as Fishmen. However, not all sea beasts are violent animals, as some are peaceful in nature and are classified as Type A creatures, "Big Friendly". Some sea beasts are actually a bit more intelligent than they let on, as seen with a small community of sea beasts in the New World that established a small town underwater and even have law enforcement.
Types of Sea Beast
Sea Cow (海牛 Kaigyū?) is a sea creature that acts, resembles, and sounds like a normal cow; however, it is apparently a carnivore. It seems to have no problem staying on land since Arlong's crew pulled it right through Gosa Village without damaging it. Oddly, sea cows are classified as type "A" creatures "Big Friendly". They may be a pun on real-life dugongs and manatees, which are commonly referred to as "sea cows". The first and so far only one that appeared is Mohmoo, who first appeared in Chapter 73 and Episode 31 as part of the Arlong Pirates.
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 Keimi and Pappug and spat them out shortly after being rescued. It first appeared in Chapter 193.
Sea Cats (海ネコ Umi Neko?, Cat Sharks in the 4Kids dub) are huge, cat/whale creatures that live near the shores of Alabasta. They are classified as a type "A" creature, being "Big Friendly". A huge sea cat was seen by the Straw Hat Pirates before coming to Alabasta. Vivi stated that it is a legendary animal that is a sacred to Alabasta, and must not be harmed. In the 4kids dub, she told them the "Cat Shark" was an endangered species. Their name is a pun on the Black-tailed Gull, which its name kanji is literally translated as "Sea Cat". Their appearances resemble that of a maneki-neko. They first appeared in Chapter 157 and Episode 92.
A Sea Kitten was later seen in Solo Journey of Jinbe, Knight of the Sea, where it became lost and could not find its way home. Jinbe then escorted the little sea kitten back to its underwater hometown.
The Sea Monkeys (シーモンキー Shī Monkī?) are sea creatures resembling monkeys. 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 first appeared in Chapter 303 and Episode 207.
In the anime, it is a shapeshifting animal and first appeared in Episode 382. 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 Keimi and Pappug. The first one debuted in Chapter 490 and Episode 385.
The Sea Lion (海獅子 Umi Jishi?) is a sea monster that lives in the depths of the ocean. One was seen in the New Fishman Pirates' army of sea monsters just outside Fishman Island in Chapter 607 and Episode 526, which was later ridden by the Flying Pirates during their attack at Ryugu Kingdom. After the New Fishman 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". It is 55m (180' 5") by estimate in length.
The Sea Bear (海熊 Umi Guma?) is a sea creature that lives on the ocean floor. The only one seen is under the command of the New Fishman Pirates, and it appeared first alongside the Sea Lion and many other sea beasts under the crew's command in Chapter 607 and Episode 526. It was on a rampage, but stopped when some fishmen tried to frighten it with fire. The kanji for sea bear can mean "fur seal" in Chinese, which the sea bear somewhat resembles. It is 50m (164') by estimate in length.
The Sea Giraffe (海キリン Umi Kirin?) is a large sea beast resembling a giraffe that lives deep in the ocean floor. One was under the ownership of the New Fishman Pirates where it served as Hammond's steed and usually traveled and fought alongside its fellow sea beasts. It was used during the New Fishman Pirates' invasion of Fishman Island. After the defeat of the New Fishman Pirates, the sea beasts went on a rampage and destroyed everything in their way, as read by Jinbe on the Newspaper. It was first seen in Chapter 607 and Episode 526.
The Sea Elephant (海ゾウ Umi Zō?) is a massive aquatic pachyderm that lives in the depths of the sea. One sea elephant was one of the many sea monsters under the control of the New Fishman Pirates. The sea elephant and its fellow sea beasts were used during the New Fishman Pirates' attack on Fishman Island. After the New Fishman Pirates were defeated and arrested, the sea elephant and its fellow sea beasts went on a spree of destruction. It was first seen in Chapter 607 and Episode 526. The sea elephant is a visual pun on real-life "sea elephants".
The Sea Rhinoceros (海サイ Umi Sai?) is a sea beast that lives deep in the sea. A sea rhinoceros was seen among the New Fishman Pirates' army of sea monsters just outside Fishman Island in Chapter 607 and Episode 526, and were later ridden by the Flying Pirates during their attack on Ryugu Kingdom. After the New Fishman Pirates were subdued, the sea rhino and its fellow sea beasts went on a rampage of destruction.
The Sea Gorilla (海ゴリラ Umi Gorira?) is a sea monster resembling a gorilla that lives in the depths of the ocean. One was seen as part of the New Fishman Pirates' army of sea beasts just outside Fishman Island in Chapter 607 and Episode 526, and were later used by the Flying Pirates during their attack on Ryugu Kingdom.. After the fall of the New Fishman Pirates, the Sea Gorilla and a few of the sea beasts went on a spree of destruction.
A Sea Squirrel is smaller species of sea beast. One was under the ownership of the New Fishman Pirates. It was used by Hammond to attack Luffy, but the beast was easily subdued by Luffy's Haoshoku Haki.
A Sea Leopard was one of New Fishman Pirates' sea monsters. After beginning their conquest of Fishman Island, the New Fishman Pirates released it along with their other Sea Beasts to aid them in battle.
Sea Pig (海ブタ Umi Buta?) is an ingredient for one of the Attack Cuisine dishes, Hormone Soup with Sea Pork. The name is 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.