Basculin is a green aquatic Pokémon that comes in three 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 three forms of Basculin is the color of the stripe that separates the black and green on their body. All 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.
Red- and Blue-Striped Basculin are extremely hostile, especially towards their opposite form. They generally start fighting each other on sight for territory. However, on rare occasions, one form may swim with the other's school. Basculin is full of vitality and reproduces quickly. When a school of Basculin appears in a lake, Pokémon other than Corphish and Crawdaunt flee. It competes with Carvanha for food.
Basculin has often been referred to as "the thug of the lake" or "Thug of the River", and uses its jaws, which are strong enough to leave bite marks in iron sheets, to devour everything in its way. However, despite their fierce reputation, Basculin become prey to stronger organisms, such as Bombirdier and Dondozo. Basculin were once also consumed by humans, and were considered very delicious. The meat of Blue-Striped Basculin has an inoffensive, light flavor, while the meat of Red-Striped Basculin is fattier, and was more popular with the youth. Basculin is a popular Pokémon with anglers due to its resistance. Many people release Basculin into lakes illegally, causing its population to grow abundant.
In Hisui, White-Striped Basculin has a similar appearance to the other two forms of Basculin. Despite its descriptor, the stripes above its eyes are actually a very pale shade of light blue, as are its irises. Its pectoral fins are smooth like Blue-Striped Basculin, but larger. Its dorsal fins are spiky and it has wide eyes, like Red-Striped Basculin. Its lower jaw is smaller than the jaws of either Red- or Blue-Striped Basculin, and both its lower and upper jaws have beak-like protrusions at the front. White-Striped Basculin is considered a regional form by technicality due to similarities on what classifies as one.
Unlike the other two forms, White-Striped Basculin are known to be gentle. They are known to travel upstream; the journey is often perilous for the species, and many of them don't survive. As a result, the souls of the deceased White-Striped Basculin are often left behind in the waters of Hisui. However, when an injured White-Striped Basculin survives, it can unite with the souls in order to achieve the unique evolution 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.
Red- and Blue-Striped Basculin share their category with Zweilous. They are both known as the Hostile Pokémon.
Despite the form being called a White-Striped Basculin, its Pokédex entry written by Professor Laventon, along with Pokémon HOME, classify it as a Hisuian Form.
In the TCG, Basculin cards featuring the White-Striped Form are called Hisuian Basculin.
White-Striped Basculin is the only regional form whose type is exactly the same as its original form.
Basculin is based on a bass, most likely the largemouth bass due to its greenish color, pattern of horizontal splotches, and vigorous resistance when being fished. It is likely also inspired by the piranha, which has historically been viewed as violent. Its Red-Striped and Blue-Striped variations that are hostile to each other are 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 Vol. 205, 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 number of fish in the region, and it was also based on a bass since there are wild bass in New York.
While Basculin's White-Striped Form may likewise be inspired by a bass, its design is also reminiscent of the salmon or the related itou (Sakhalin taimen), due to its behavior and the shape of its mouth resembling the kypes that develop in male salmon.
The selection of colored forms being red, white, and blue may reference the national flag of the USA, where Unova, the region Basculin was first introduced, is based.
Basculin may be a combination of bass and masculine.