Which Pokémon game first introduced double battles as a regular battle format?

Pokémon Ruby and Sapphire first introduced double battles as a regular battle format.

Released for the Game Boy Advance in Japan in 2002, Ruby and Sapphire allowed two Pokémon on each side to fight simultaneously. This changed battle strategy because moves could target allies, opponents, or both sides, depending on their effects.

The games are set in the Hoenn region and form the core releases of Pokémon's third generation. They also introduced abilities, natures, Pokémon contests, and 135 new species, bringing the total National Pokédex count to 386 at the time.

Earlier games included single battles as their standard format. Later titles expanded the possibilities with triple battles, rotation battles, and other special rules, but double battles remained the foundational multi-Pokémon format in the main series.