Double battles, featuring two active Pokémon per side, debuted in Pokémon Ruby and Sapphire.
The format changed the traditional one-on-one structure by requiring players to consider four Pokémon on the battlefield at once. Moves could target one opponent, an ally, multiple combatants, or every Pokémon affected by the move's rules.
Ruby and Sapphire introduced the format when they launched for Game Boy Advance in Japan on November 21, 2002. They also added Pokémon Abilities and Natures, which gave individual Pokémon more specialized battle roles and influenced how teams were built.
Later games expanded the idea with multi battles, in which two trainers share a side, and other formats such as Triple Battles and Rotation Battles. Those are related but distinct systems, so the original two-versus-two format is specifically called a Double Battle.