Hydreigon is a powerful Dark and Dragon type Pokémon. It's a great Pokémon to add to your roster, especially in Pokémon Go. Capturing the unique creature is a lot more complicated than simply acquiring it. Hydreigon is the final evolution of Deino, so you need to start with the first form before getting the third and final form of Hydreigon.
You have the chance to encounter Deino in the wild and capture him in 10 kilometer eggs. For those who are fully invested in trying to catch this Pokemon, you might want to clear out all of your incubators and start dumping all of your 10 kilometer eggs into them. Because it's a Dark and Dragon type, you have a chance to encounter it during the night, in movie theaters, in towns, and on golf courses. When you locate Deino at a particular location, try to find a nearby PokéStop to use a Lure, and try to capture more.
You need 25 Deino candy to evolve it into Zweilous, then 100 candy to evolve it into Hydreigon. It has a max CP of 3, an attack of 625, a defense of 256, and a stamina of 188. It has an incredible attack power threshold, but is slightly lacking in defense, so you might not want to use it as your first option in a battle.
For those considering using Hydreigon against other trainers, you want him to know Dragon Breath, Dark Pulse, and Dragon Pulse. He can learn the flash cannon, but given the alternatives, you're better off giving that up as a suitable option. When battling Pokemon in Raids or Gyms, you want it to know Bite and Dark Pulse, Dragon Breath and Dark Pulse, or Dragon Breath and Dragon Pulse. Dragon's Breath and Dark Pulse might be the most appealing option as they cover two different types.
When it comes to adding a Hydreigon to your collection, it's all about taking the time to be patient and keep researching them. If you know you can't locate a Deino in your town, try acquiring 10-kilometer Eggs from PokéStops to make sure you still have a chance of capturing it.