Pokemon with hidden abilities have an encounter rate of 0% unless certain circumstances are met. These are the circumstances.
Certain event Pokémon may have their Hidden Ability.
In Generation V, all Pokémon encountered in Entree Forest (i.e. obtained from the Pokémon Global Link, either via the Pokémon Dream World or a Global Link promotion) will always have their Hidden Ability, if they have one.
Pokémon obtained from the Pokémon Dream Radar will have their Hidden Abilities.
In Pokémon Black and White, some overworld Pokémon have their Hidden Ability:
The five Darmanitan in the Desert Resort.
The Musharna in the Dreamyard on Fridays.
Any Pokémon found in Pokémon Black 2 and White 2's Hidden Grottos have their Hidden Ability.
In Pokémon Black 2 and White 2, some overworld and gift Pokémon have their Hidden Ability:
On Mondays, a male Jellicent at Undella BayB2 or a Braviary on Route 4W2.
On Thursdays, a Mandibuzz on Route 4B2 or a female Jellicent at Undella BayW2.
A Deerling given as a gift on Route 6.
Yancy/Curtis will trade the player a specific Pokémon with a Hidden Ability after entering the Hall of Fame and riding the ferris wheel with them.
Any Pokémon encountered in a Horde Encounter may have their Hidden Ability.
Pokémon encountered in the Friend Safari have approximately a 1 in 3 chance of having their Hidden Ability if the Friend to whom the Friend Safari corresponds has been recognized as a Friend in the PSS (i.e. if both players have been on the PSS at the same time).
Hidden Pokémon have a chance of having their Hidden Ability, which increases as the Search Level for that species increases.
In Generation VII, Pokémon that appear as the tenth or later Pokémon in an SOS Battle have a chance of having their Hidden Ability, which increases as the chain lengthens.
All Pokémon transferred from the Virtual Console releases of the Generation I games via Poké Transporter have their Hidden Ability.
By the way, Staraptor is usually better than Cinccino.