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