Basculin has two different appearances: Red-Striped Form, which has a red stripe and irises, with a round eye and spiky fins; and Blue-Striped Form, which has a blue stripe and irises, with a slit eye and smooth fins.
Basculin is a green aquatic Pokémon that comes in two forms. The bodies of both the Red- and Blue-Striped Forms are the same. They have a green, ovoid body, tapering off in the rear to a trapezoidal tail fin. A thick black stripe covers the top of their body from mouth to tail and a white, dorsal fin with four protrusions resembling spikes. Their lower jaw juts ahead of their upper jaw, with a small indent near the edges of their mouth. Both forms have three black dots on both sides of their body, behind their pectoral fins.
The main difference between the two forms of Basculin is the color of the stripe that separates the black and green on their body. Both forms of Basculin have fins in the same places, with one on top of their body ahead of the tail fin, one below the body ahead of the tail fin, and two side fins. The Red-Striped Form has jagged fins that resemble tridents, whereas the Blue-Striped Form has smooth fins all around. The Red-Striped Form have circular, red eyes, while the Blue-Striped Form have narrower, blue eyes.
Both forms of Basculin are extremely hostile, especially towards their opposite form. Red- and Blue-Striped Basculin generally start fighting each other on sight for territory. However, on rare occasions, one form may swim with the other's school. When a school of Basculin appears in a lake, Pokémon other than Corphish and Crawdaunt flee. Basculin has often been referred to as "the thug of the lake". Basculin uses its jaws to devour everything in its way. However, despite their fierce reputation, they become prey to stronger organisms. Basculin are also consumed by humans and considered very delicious. The Blue-Striped Form is a common food source with an inoffensive, light flavor. The Red-Striped Form was often placed on dinner plates in the past, and it is more popular with the youth. The meat of the Red-Striped Form is said to be fatty. Basculin is a popular Pokémon that anglers enjoy fighting while fishing, and they often release it back into the lake illegally due to how abundant the Basculin are. It competes with Carvanha for food. In Hisui, the deceased souls of Basculin can unite with a living Basculin in order to evolve into Basculegion.
In Pokémon Black and White, blue-striped Basculin have the same two standard possible Abilities as red-striped Basculin—Reckless and Adaptability (and is listed as such in Pokédex 3D); however, the blue-striped Basculin that can be obtained via in-game trade in White has the Ability Rock Head. In Pokémon Black 2 and White 2, wild blue-striped Basculin's two standard possible Abilities are Rock Head and Adaptability, while red-striped Basculin's stay the same (and are listed as such in Pokédex 3D Pro). However, blue-striped Basculin bred in Pokémon Black 2 and White 2 have Reckless and Adaptability as their two standard Abilities. When a blue-striped Basculin with Reckless is transferred from Generation V to Generation VI, its Ability is changed to Rock Head.
Basculin shares its category with Zweilous. They are both known as the Hostile Pokémon.
Basculin's Pokédex entries in Pokémon Black 2, White 2, X, and Omega Ruby describe the species as being very tasty, one of the rare instances that hint at Pokémon being used for food by humans.
Basculin were most likely inspired by a bass or piranha, the latter of which has historically been viewed as violent. Its red and blue variations that are hostile to each other is similar to Siamese fighting fish, which come in a variety of colors (including red and blue), and are extremely aggressive and territorial to others. It could also be compared to street gangs, where groups of affiliated people often fight rival groups over their own territory.
According to interviews with Ken Sugimori in Nintendo Dream, Basculin was created late in development when they realized there were few new "standard" fishlike Pokémon in Unova, the only others being Alomomola, Stunfisk, Frillish, and Jellicent. It was given two forms to make up for the small amount of fish in the region, and it was also based on a bass since there are wild bass in New York.