Hidden Grotto Encounters .

In Pokémon Black 2 & White 2, a new feature called Hidden Grottoes were introduced. In these spots, besides random item spawns, players get to encounter Pokémon that have their Hidden Ability, which cannot be normally found in standard grass/cave/water encounters in the games. The levels varies, depending on which Grotto the Pokémon was encountered in. It is important to note that any kind of Hidden Grotto encounter cannot be shiny.

Besides the standard grotto encounters, the in-game mechanic Funfest Missions also allows for different Pokémon to be encountered, though all but one of such mission are local wireless missions.

This page hosts files of the various different species that can encountered in the standard grotto, as well as Funfest Mission grottoes.
To be completely transparent, the Funfest grotto encounters were obtained using RAM manipulation, as detailed here. The only reason why this is done and why the files are included, is because it appears the encounters generated are indistinguishable from the actual Funfest mission.

Poliwag has been removed, because there's an announcement that claims the announcement for Poliwag was a mistake, and wouldn't be included.