3 species have Stench as a hidden ability, 6 as a normal one

Effect, in PokéAPI's words

“Has a 10% chance of making target Pokémon flinch with each hit.”

As a hidden ability

Species	Slot	Note
Gloom	Hidden	Since Generation V
Koffing	Hidden	Since Generation V
Weezing	Hidden	Since Generation V

As a normal ability

Species	Slot	Note
Grimer	Normal
Muk	Normal
Stunky	Normal
Skuntank	Normal
Trubbish	Normal
Garbodor	Normal

Elsewhere in PokéAPI

1 more holder without a page here

Alternate forms and other species that PokéAPI lists with Stench.

Garbodor Gmax