Zweilous is a dark-blue, quadruped 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. As a result, the heads often turn eating into a competition. Whichever head eats more than the other gets to be the leader. Because of this, Zweilous always eats far too much. It is a nomadic Pokémon; it migrates once it has eaten all the prey in an area.
Of all the Pokémon that evolve by level alone, Zweilous can evolve into its final form, Hydreigon, later than any other Pokémon, evolving at level 64.
According to interviews with Ken Sugimori in Nintendo Dream, Zweilous and its evolutionary family were originally intended to be cyborg dragons with aspects of tanks. The 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 Basculin. They are both known as the Hostile Pokémon.
Zweilous may be a combination of zwei (German for two) and jealous (as the heads always fight over food). It may also derive from zealous, which reflects Zweilous's voracious appetite and intensely competitive nature.
Dihead is a combination of di (Greek for two) and head.