Hydreigon did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
Hydreigon, known in Japan as サザンドラ (Sazandora), is a powerful Pokémon of the Dark/Dragon type. Hydreigon evolves from Zweilous at level 64 and is thus the final evolution of Deino. It is the pseudo-legendary Pokémon of Unova, as well as the most savage one of them all.
Hydreigon is a draconic, pale blue Pokémon with three heads: two in place of hands, and a middle one which resembles the head of a serpent. All three heads and necks are covered in a black fur-like substance with a purple underside and ragged frills. The fur on the limb-like hands resemble sleeves and seem to resemble hands in itself, while the fur around the middle head resembles a crown-like collar. All of Hydreigon's heads have pitch-black eyes, but the middle head has a fierce-looking purple pupil. It is also the only head with fangs; the other two heads are toothless.
Hydreigon's chest and back is covered in the same substance as the heads, and so are its six ragged, slender wings. Two purple, stripe-like patterns are found on its belly, and another one on its tail, which ends with a black, ragged tip. It also has two feet, which appear to be atrophied due to the lack of usable features and overall raggedness.
Hydreigon is a very savage Pokémon that usually travels through the sky with its wings. Literally anything that moves seems like either an enemy or a threat to Hydreigon, and if it spots a moving individual, it responds by attacking it. Wild Hydreigon appear to consume and destroy anything it encounters, with makes it a huge threat.
Interestingly, only the middle head appears to be sentient, as the two other heads appear to lack a brain and are directly controlled by the middle head.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
Of all the Pokémon that evolve by leveling up, Hydreigon's pre-evolution takes more levels than any other Pokémon. Zweilous evolves into this Pokémon at level 64, which is by far the highest level for a Pokémon to evolve on.
Hydreigon shares its category name with Sharpedo: both are known as the Brutal Pokémon.
The original pseudo-legendary Pokémon of Unova was going to resemble a cyborg dragon with references to tanks. This idea was scrapped and turned into this Pokémon, but traces of the original idea can be found back in the form of Hydreigon's belly pattern, which resemble caterpillar tracks.
Hydreigon's base origin derives from the mythological creature known as the Yamata no Orochi, a serpent with eight heads and red eyes. Unlike the Orochi, however, Hydreigon has only three heads, because it looks less confusing. The fact that it has three heads makes it also a reference to the Hydra and the Zmey Gorynych. All three draconic creatures are antagonistic that cause homicidal disasters, which also may have inspired the Dark typing Hydreigon has.
Hydreigon's name is a combination of Hydra, Drei (German for three) and dragon.
Sazandora comes from 三 san (three) as well as dragon or hydra. It may also come from Pandora, who according to Greek mythology, accidentally released all the evil in the world by opening a box it was sealed in.