Zweilous is a dark-blue, quadrupedal Pokémon with two heads and black fur covering its necks and back. Its front legs are muscular with two claws on each foot, but its rear legs are less defined and have two small, backward-pointing toes without claws. There are tufts of fur over its heads with rounded points at the top. It has two thin wings on its back, each ending in two points. The fur stops at the chest and opens up to fuchsia striped bands on the side of its legs. Though it appears to lack eyes, it is unknown if Zweilous is blind.
Zweilous has the vigor and energy to eat and bite constantly without stopping. Its heads dislike each other and are very hostile. Each head has their own likes and dislikes. As a result, the heads often turn eating into a competition, each head trying to claim leadership by eating more than the other. Because of this, Zweilous always eats far too much and is covered in scars, even without opponents to fight. This allows Zweilous to become stronger without any allies helping it. It is a nomadic Pokémon; it migrates once it has eaten all the prey in an area. However, Zweilous generally makes its habitat in caves.
Of all the Pokémon that evolve by level alone, Zweilous can evolve into its final form, Hydreigon, starting at the highest level, evolving at level 64.
According to interviews with Ken Sugimori in Nintendo Dream, Zweilous and its evolutionary family were originally intended to have aspects of tanks. These Pokémon were scrapped until late in development, when they were revived and remade to be based on Yamata no Orochi. The tank tread-like markings on Zweilous's underbelly are a remnant of this design.
Zweilous shares its category with Red- and Blue-Striped Basculin. They are all known as the Hostile Pokémon.
Zweilous and Deino are the only Pokémon who have Hustle as their only Ability.