Some Pokémon have more than one possible Ability. In this case, the chances of obtaining a Pokémon with a particular Ability are equally likely. These Pokémon are known as "dual Ability" Pokémon; similarly, those with only one are known as "single Ability" Pokémon. Some single Ability Pokémon may have Abilities unique to themselves or their evolutionary lines: a signature Ability. However, in the Pokémon Mystery Dungeon series, dual Ability Pokémon have both Abilities at the same time.
Pokémon that evolve and have a certain Ability in their pre-evolution (dual-Ability Pokémon only), but evolve into a dual-Ability Pokémon upon evolution (such as Poochyena with Run Away or Quick Feet evolving into Mightyena with Intimidate or Quick Feet), the Pokémon will keep its Ability from its pre-evolution upon evolving (so a Poochyena with Quick Feet in Generation IV would keep its Ability upon evolving into a Mightyena, while one with Run Away would evolve into a Mightyena with Intimidate instead). If the Pokémon is a single-Ability Pokémon, then it will still have a 50% chance for each if its evolution is a dual-Ability (dependent on IVs).
In Generation IV, several Pokémon obtainable in Generation III gained new Abilities, becoming dual-Ability Pokémon. These Pokémon will keep the Ability they had in Generation III unless they evolve, when their Ability will be recalculated. All Generation III Abilities were maintained, with the exception of the unused Cacophony. The only Ability listed after Cacophony in the game data, Air Lock, was renumbered, and the new Abilities followed after.