Momoo

Standing 36 meters tall and weighing roughly 16 tons, this creature carries a rounded cow-like face topped with horns and a broad snout pierced by a golden ring. Green blotches dot his hide, and a layer of fish-style scales runs from his throat down to his belly. Those scales are really hardened skin that shields his organs the way armor would. Although his form resembles a seal and bears such scales, he is still grouped among the mammalian Sea Beasts. His fins, besides aiding his swimming, conceal retractable bones that can jut out into claw-like points.

Simple in mind like most pirate pets, Momoo devotes nearly all his attention to eating or to obeying whoever he treats as a master. His bulk and carnivorous appetite make him willing to devour any human who wanders too close, yet anything stronger than himself sends him cowering. Repeated thrashings from Luffy and Sanji left him terrified of the Straw Hat Pirates, a dread that outlasted the two-year gap. Spotting the crew again on the way to Fish-Man Island, he bolted in panic until the Caribou Pirates clubbed his skull and forced him back.

Arlong and his crew supposedly came across the beast somewhere along the Grand Line and brought him to the Conomi Islands, where he was kept in an underwater cavern near Arlong Park. Hatchan, who could call him with a loud trumpeting blast, looked after him and occasionally unleashed him as muscle, with one such rampage flattening Gosa Village entirely. Weeks afterward he trailed cooking smoke to a ship carrying Luffy, Sanji, and Yosaku, earning a beating from both pirates before being strapped to the vessel and made to haul it toward Arlong Park. Exhaustion sent him veering off course into Cocoyasi, where he knocked himself out against the shore. When the crew stormed Arlong Park, Hatchan summoned him once more, and only Arlong's sheer force of will overrode his fear before Luffy spun him around and swung him as a club against the Fish-Men. Long after Arlong fell, Momoo drifted toward the Sabaody Archipelago, was caught by the Caribou Pirates, and served as their hull-mounted navigator until the Kraken smashed their ship and set him loose.