HA - Hidden Ability
Beginning in Generation V, most Pokémon were given a Hidden Ability. A Pokémon with its Hidden Ability will retain its ability slot upon evolution, unless the evolved form has no Hidden Ability.
To understand which ability is considered a Pokémon’s Hidden Ability, please use
Google with the query
hidden ability <pokemon> replacing
<pokemon> with the
Pokémon name that you’re looking for. A comprehensive list is beyond the scope
of this FAQ.
Getting a Hidden Ability
Hidden Abilities are available in the game, and are obtained via finding Pokémon in the wild with them. They have a 1% chance of appearing, and it is recommended to have a Trace ability Pokémon to make HA hunting the most efficient possible.