Which Pokémon games first introduced the Ability battle mechanic?

Pokémon Ruby and Sapphire first introduced the Ability battle mechanic. These Game Boy Advance games added innate Abilities that could affect battles in different ways.

An Ability can grant effects such as immunity to certain moves, protection from status conditions, or increased power for a particular type of attack. The feature gave individual Pokémon additional strategic identity beyond their species, moves, and statistics.

Ruby and Sapphire were released in Japan on November 21, 2002, as the first main games of Pokémon's third generation. They also introduced double battles, Pokémon Contests, natures, Secret Bases, and 135 new Pokémon.

Pokémon Emerald used the same generation and included the Ability system, but it was not the introduction point. Gold and Silver introduced held items and breeding, while Ruby and Sapphire added Abilities as a distinct core battle mechanic.