Dressrosa Fighting Fish
These fish live in the New World portion of the Grandline, specifically around Dressrosa! They are easily identified by their bull-like horns, sharp teeth, black scales, and red/yellow eyes. They also have two spineless pectoral fins, a spineless dorsal fin, and a spined caudal fin!
They are related to goldfish, making them a subspecies of the carp family(Although it is heavily debated on what genus of carp.) It spends most of it's time close to the surface attacking anything and everything that moves, even large-sized boats that try to cross between Dressrosa and Greenbit.
Due to their nature, there are currently no natural predators that hunt fighting fish! Because of this, the bridge connecting to Greenbit is unusable and they are used in the Corrida Coliseum as entertainment. They appear to have a social hierarchy, though it's hard to know how the structure is determined.
Some sources say these fish very suddenly arrived in Dressrosa about 200 years ago, and the sudden arrival rings true to me due to the nature of the bridge and stories of Greenbit before their arrival. It is unclear why they chose Dressrosa or what possible rapid adaptation might have taken place due to the new environment.
Personal note: It is far too unsafe to dive here. I made a horrible decision, please do not repeat.