Which Pokémon games introduced Abilities to battles?

Pokémon Ruby and Sapphire introduced Abilities to Pokémon battles.

Released for the Game Boy Advance, Ruby and Sapphire were the first-generation-three main-series games and debuted in Japan in late 2002. An Ability is an innate effect associated with a Pokémon species or individual Pokémon that can influence battles and exploration.

Examples include Levitate, which grants immunity to Ground-type moves, and Intimidate, which lowers an opposing Pokémon's Attack when the Pokémon enters battle. These effects added another layer of strategy beyond moves, types, statistics, and held items.

Ruby and Sapphire also introduced double battles, Pokémon Contests, Natures, and 135 new Pokémon. Abilities are sometimes confused with Natures because both were new in this generation, but they are different mechanics: Abilities create special effects, while Natures primarily modify statistics and preferences.