Hydreigon is based on the Yamata no Orochi, a type of Japanese dragon. It could also be based on the Hydra of Greek mythology, as it's name implies.
Hydreigon has received very positive reception for sporting a unique Type combination, while also having a diverse movepool.
It is reputable for being Ghetsis' most lethal, but infamously underlevelled Pokémon on his team in all of Generation V's main series games.
Hydreigon has the highest base Sp. Atk stat out of any Pseudo-Legendary Pokémon so far, enabling it to take advantage of moves like Fire Blast, Focus Blast, Flash Cannon, and Earth Power, as well as gaining STAB from attacks such as Draco Meteor, Dragon Pulse, and Dark Pulse. Like most Pseudo-Legendaries, it has strong stats. Although, it is rather balanced in stat distribution compared to most Pseudo-Legendaries.
However, Hydreigon is easily outsped by most sweepers, making it rather little difficult to use, unless equipped with a Choice Scarf. In Generation VI, it became the subject of humour due to it's 4x weakness to the recent Fairy Type. Many fans have jokingly shipped Hydreigon with Sylveon, due to a promo art for X & Y showcasing both Pokémon confronting each other.