They Live on the Land and Cover Their Bodies with a Poisonous Film
In ancient times, Wooper lived underwater in the Paldea region. But it seems after losing in a struggle for territory, they began living in bogs on land.
To keep from drying out while living on land, they began to cover their bodies with a poisonous film.

Powerful Poisonous Liquid Emits from Their Tough Gills
These Wooper’s gills have hardened thanks to living on land for so long. Their bodies are heavy and they move slowly, but they can protect themselves by shooting powerful poisonous liquid from their gills.
You may see poisoned Pokémon in areas where these Wooper live.