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