Hydreigon is a large three-headed draconian Pokémon. It is mainly blue with black scruffy fur covering its arms and neck. The black fur that covered up its eyes as a Deino and Zweilous has disappeared, creating a frill-like appearance and is now magenta in color. Instead of having one wing-like appendage on each side of its body, it now has three wings on each side of its body that allow it to fly. On the end of each arm is a smaller blue head with eyes and a mouth, but no brain. Zweilous uses both its main head and its head-like hands to voraciously devour its food. After Zweilous evolved, its tail has grown longer and has magenta scales on the underside as well as a black tip on the end. Its round, plump belly has two rows of magenta scales just like it had when it was a Zweilous. Its abdomen seems to have grown plumper and fuller as a result of eating so much food as a Zweilous.
Hydreigon is said to be a violent and brutal Pokémon that devours and destroys everything it sees. However, it appears to be harmless and loyal when raised and owned by a skilled trainer. As seen in the anime, it will allow its trainer to ride upon its back and fly.
In Pokémon Mystery Dungeon: Gates to Infinity Hydreigon is a wise and very kind Pokémon who represents and embodies the Voice of Life; he is endearingly quirky and will assist the hero throughout the game.
Hydreigon seems to resemble Salamence because both of their two pre-evolutions have weaknesses until their pre-evolutions evolve into them.
Hydreigon's pre-evolutions are blind and unable to fly until Zweilous evolves into Hydreigon.
Salamence's pre-evolutions are unable to fly until Shelgon evolves into Salamence.
The frills around Hydreigon's heads were covering its eyes in its previous forms.
Ghetsis' Hydreigon is under-leveled, being at level 54 despite Zweilous not evolving until Level 64. Due to this fact, as well as it knowing vast coverage moves such as Surf, Fire Blast, and Dragon Pulse, Ghetsis' Hydreigon has become infamous, being referred to as the hardest single Pokémon to beat in the entire game.
Hydreigon is one of the hardest to obtain due to the high level it evolves at, beating even Dragonite and Tyranitar and because Deino, its first form, can only be obtained in Victory Road and does not appear very often.
However it has become relatively easier to obtain since Pokémon Black 2 and White 2 now feature Zweilous in Victory Road though it also does not appear very often.
The Pokédex says that the heads on Hydreigon's arms lack brains, but it still uses them to eat and destroy things like hands since they're probably connected to the main head's brain and may possess "pseudo-brains" similar to the tentacles of an octopus.
Hydreigon's name may also come from the word "dragonfly" because its wings resemble those of a dragonfly.
Hydreigon also somewhat resembles Giratina in origin form, given that both Pokémon have six tentacle-like wings.
Hydreigon is the only Pokémon to have heads for hands, though Scizor's Pokedex entry in Gold and HeartGold say its pincers resemble heads.
Hydreigon can learn at least one move of each type except Grass-type and Fairy-type moves.
The patterns on Hydreigon's sides bear resemblance to tank treads.
It shares species with Sharpedo both known as the Brutal Pokémon.
Hydreigon is based on a multi-headed dragon, such as the Hydra of Greek mythology.
Hydreigon's name came from "Hydra", a Greek mythological dragon with many heads, "Drei", the German word for the number 3, and "Dragon."