In Pokémon Black & White, a third hidden ability was introduced.
In D/P/Pt, there is a 0% chance for a Pokemon to have their Hidden Ability, as Hidden Abilities did not exist yet.
However, in later games, the chance of getting a Hidden Ability Pokemon is different, depending on what method you use, I will list the chances (If there is a confirmed chance) for each method below.
A Pokemon found in the Friend Safari has a 1 in 3 chance to have its Hidden Ability, if the friend that that Friend Safari is from has been on the PSS at the same time as you at some point.
All Pokemon that are found in B2/W2's Hidden Grottos will have their Hidden Ability.
The male Jellicent found on Mondays in B2 at Undella Bay will always have its Hidden Ability.
The Mandibuzz found on Thursdays in B2 at Route 4 will always have its Hidden Ability.
The female Jellicent found on Thursdays in W2 at Undella Bay will always have its Hidden Ability.
The Braviary found on Mondays in W2 at Route 4 will always have its Hidden Ability.
The Deerling gifted to you on Route 6 in B2/W2 will always have its Hidden Ability.
Yancy/Curtis will trade you a Hidden Ability Pokemon in B2/W2.
The five Darmanitan found at the Desert Resort in B/W will always have their Hidden Ability.
The Musharna found in the Dreamyard in B/W will always have its Hidden Ability.
All Pokemon obtained from the Pokemon Dream Radar will have their Hidden Abilities.
All Pokemon that can have a Hidden Ability will have their Hidden Ability when obtained from the Dream World or a Global Link Promotion.
Event Pokemon might have their Hidden Ability.
Hidden DexNav Pokemon have a small chance to have their Hidden Ability, this chance increases as more of that same species of Pokemon is encountered.
It was determined here there is a 4.8% chance of Pokemon encountered in hordes having their Hidden Ability.
Hope this helps!
Sources: This and this.