Where to Find and Catch Xerneas
From Pokemon Go Wiki
Xerneas are found and captured in Nest and Habitat locations.
Nests
Xerneas Nests are locations that consistently spawn Xerneas.
Habitats
Xerneas Habitats are spawn locations where Xerneas 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:
