Zweilous is a quadrupedal two-headed draconian Pokémon. Two black wing-like appendages and a second head have appeared on its body upon evolving from Deino. Black scruffy fur covers both heads' necks and eyes. It has two claws on each front leg, as well as a stubby tail and two feather-like "horns" on both of its heads. Its round, plumper underbelly has two rows of magenta scales. Zweilous' belly is rounder and wider due to the fact that it is a voracious Pokémon that is constantly eating; its heads compete for food, so it eats more than necessary.
Zweilous' two heads do not get along and will often fight and compete with one another. They are especially competitive over food, so Zweilous will often eat too much because each head wants to eat more than the other. Whichever of Zweilous' heads eats more becomes the dominant head.
Zweilous is the evolved form of Deino as of level 50, and evolves into Hydreigon at level 64.
Two Zweilous made their debut in A Village Homecoming!, where one of them went on to evolve into a Hydreigon.
Zweilous evolves at the highest level of any Pokémon (level 64).
It seems to have a pair of wings growing or developing on its back. This makes sense because its evolved form Hydreigon has wings.
Despite Zweilous's two heads having separate minds and brains Hydreigon is explicitly stated to only have one head. Whether Zweilous loses one if its brains or if the two fuse together is unclear.
Basculin and Zweilous are the same species, both being Hostile Pokémon.
No Trainer in the Generation V games has Zweilous. It shares this trait with Rufflet, Vullaby, Mandibuzz, and Ferrothorn.
Due to the fact that Hydreigon has visible eyes and a "frill" around its neck, it can be presumed that Zweilous and Deino have eyes underneath their "hoods". It is also possible that Hydreigon's "hands" were originally Zweilous's two heads.
Since Zweilous has only 2 wing-like appendages, it cannot fly, as opposed to Hydreigon, who must have 6 wings in order to fly properly. In the anime it has been shown capable of limited flight, however.
Zweilous is based on a two-headed dragon. Zweilous is possibly modeled after the Hydra in Greek and Roman mythology, and shares the multiple heads associated with the hydra. The black fur surrounding its heads resembles a hoodie, associated with some goth or emo trends, with its bangs resembling an emo fringe.
Its name appears to be combination of Zwei, the German word for the number 2 (referencing Zweilous' number of heads and its number on the evolution line) and jealous, since one head is always competing with the other. For instance if one head ate more than the other, the other would be jealous of the other. Also, Zweil might also came from Zwilling, the German word for twin since Zweilous has two identical heads.