The only in game trainers you can battle that have a Shiny Pokémon, are three trainers in Generation III in Pokémon Fire Red and Leaf Green.
In this generation, several Pokémon Battle e Trainers have Pokémon which are preset as Shiny. Additionally, in the Trainer Tower in Pokémon FireRed and LeafGreen, there are three Trainers who have preset Shiny Pokémon: the Trainer on the seventh floor in the Single Battle mode has a Shiny Meowth, the pair of Trainers on the first floor in the Double Battle mode have a Shiny Espeon, and finally, the second Trainer on the fourth floor in the Knockout Mode has a Shiny Seaking. No Trainers have preset Shiny Pokémon in the Mixed Mode.
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