Which Pokémon games first introduced Abilities that affect battle mechanics?

Pokémon Ruby and Sapphire first introduced Abilities that affect battle mechanics.

An Ability is a special trait assigned to a Pokémon that can influence battles or exploration. Examples include effects that protect a Pokémon from certain moves, strengthen a type of attack, or activate under specific conditions.

The feature debuted with Ruby and Sapphire, the third-generation core games for Game Boy Advance. It gave individual species a more distinctive mechanical identity and created additional strategic decisions beyond choosing moves and types.

Abilities are different from Pokémon moves, held items, and Natures. A move is selected during battle, an item is carried by a Pokémon, and a Nature primarily modifies stat growth and preferences. An Ability is an inherent trait that operates as part of the Pokémon’s data and can activate automatically or in response to a battle situation.