Detailed Information regarding Hydreigon’s Location and the strategy to encounter it in Pokémon Sun and Moon.
How to Get a Deino
Source: http://www.pokemon.jp/zukan/
Deino will appear at Ten Carat Hill when you do an Island Scan using the QR Code Scanner during Tuesdays in Melemele Island. However, once it appears, you have to capture it within the specified time limit.
How to Get a Zweilous
Deino will evolve into Zweilous at level 50.
How to Get a Hydreigon
Zweilous will evolve into Hydreigon at level 64.
How do you get hydreigon?
Just level it up to 64 and it’ll evolve into a Hydreigon as the final evolution.