Wondereye

Wondereye was a very common hack near 2003. Prior to the release of Fairy type in the latest released generation, Pokémon such as Sableye and Spiritomb had no weaknesses because they were Dark and Ghost types. This made people think... What could make these Pokémon the most powerful of all? That would be hacking. This common hack gave people opportunities to make them invincible. And the only way is Wonder Guard. People would hack the game to give Sableye or Spiritomb the ability Wonder Guard, which lets only super-effective moves to hit the Pokémon. The Pokémon Shedinja is the only pokemon that can have it without hacking, and people gave Sableye and Spiritomb this ability.

Maybe Pokémon noticed this was happening, so perhaps they made Fairy type because of that. Sableye and Spiritonb were now defeated. The Wondereye hack was a type of hack that could perhaps let you win every battle.