Where to Find and Catch Hydreigon
From Pokemon Go Wiki
Hydreigon are found and captured in Nest and Habitat locations.
Nests
Hydreigon Nests are locations that consistently spawn Hydreigon.
Habitats
Hydreigon Habitats are spawn locations where Hydreigon is likely to be encountered, but not guaranteed the way a Nest is.
Additional Tips and Information
Best Nests
If you know a really good nest or any of the nests above is really good, list them here: