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