Like Deino, Zweilous is a blue dinosaur-like Pokémon, but after evolving, it gained a second head. Black fur covers both heads' necks and eyes. Two wings have also appeared after evolving. On the end of each front leg are two claws. It has a stubby tail and two feather-like "horns" on both of its heads. Its underbelly has two rows of magenta scales.
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).
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 have 2 heads and they are twins.
It seems to have a pair of wings growing or developing on its back. This would make sense because its evolved form Hydreigon has wings.
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.
Zweilous is modeled after the Hydra in Greek and Roman mythology, and shares the multiple heads associated with the hydra.
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".
Since Zweilous has only 2 wings, it still cannot fly, as opposed to Hydreigon, who must have 6 wings in order to fly properly.