Learning Hippopotas' counters in Pokémon GO is important if you ever want to catch the shiny version or defeat the raid encounter. During the month of February, Niantic Labs announced the Pokémon GO Sinnoh region celebration.
This will increase the spawn of certain Pokémon like Hippopotas.
Hippopotas Counters Pokémon GO: Which Pokémon Should You Use to Defeat Hippopotas?
To best defeat Hippopotas in a raid, you need to use Grass, Ice, and Water-type Pokémon. Hippopotas is a Ground-type Pokémon, so he's weak to all three. If you're lucky you can catch a shiny version of Hippopotas. The best way to catch one is by hatching 7km eggs or farming Hippopotas raids. Both have a chance at becoming a shiny.
If you choose the raid, make sure to use the proper counters.