Hydreigon is mainly blue with black fur covering its arms and neck. The black fur that covered up its eyes as a Deino and Zweilous has now opened, creating a frill-like appearance and is now magenta in color. Instead of having one wing on each side, it now has three wings on each side of its body. On the end of each arm is a smaller blue head. After Zweilous evolved, its tail has gotten longer and has magenta scales on the underside as well as a black tip on the end. Its belly has two rows of magenta scales just like it had when it was a Zweilous.
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.
Hydreigon appears to be a parallel to Dragonite as it is a Dragon Pseudo-Legendary.
The frills around Hydreigon's heads were covering its eyes in its previous forms.
Ghetsis's 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's Hydreigon has become infamous, being referred to as the hardest single Pokémon to beat in the entire game.
It could be possible that Ghetsis's Hydreigon's evolution was induced by an item or by a machine.
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 Pokemon 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 forme, 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's name sounds like the element hydrogen.
Hydreigon can learn at least one move of each type except Grass-type moves.
The patterns on Hydreigon's sides bear resemblance to tank treads.