Hydreigon is a Dark/Dragon-type Pokémon introduced in Generation V. It's known as the Brutal Pokémon.
This Pokémon's name may come from hydra, a mythical Greek creature that is similar to it, drei, which stands for the number three in German, and dragon. Its Japanese name is Sazandora (サザンドラ). Both sa and zan seem to come from sa/san (三), three in Japanese, as a reference to its three heads. dora stands for multiple dragons or hydras, reference to the multiple heads hydra had in the Greek mythology. dora could also be a reference to the Japanese movie of monsters King Ghidorah.
This Pokémon seems to be based on both mythical Greek creatures hydra and Cerberus, the three-headed dog. Also, it is said that Hydreigon is the guardian to the entrance to the underworld, just as Cerberus.
It's a Pokémon that usually goes through the skies using its six wings, detecting anything that moves as an opponent to attack to. Hydreigon will only obey a Trainer it considers worthy for its respect and bravery. It has a great power, and, according to Unova mythology, a three headed Pokémon (Hydreigon) and a Pokémon with heartrending fangs (Haxorus) fought and destroyed everything in their path. Today this is not common because Pokémon Rangers have been distributed to impede those Pokémon from avancing further. This Pokémon can't have physical contact with a Haxorus, due to the things previously mentioned.
This Pokémon can be somewhat annoying, but if taken care properly and given confidence, it can be a really good Pokémon in your team, it can even sacrifice itself to save its Trainer. The heads in its hands doesn't have brain, so all the orders are given from the brain of the head in the center. If it finds a stronger opponent, it will fight and try to win in any possible way.
This Pokémon is hard to raise due to the high levels it evolves in. However, it may be worth, due to it being a really strong Pokémon.
So, what Pokémon would you like me to do next?