Fairy was introduced in Pokémon X and Y as the first new Pokémon type since Gold and Silver.
The Fairy type debuted with the sixth generation in 2013. It applied to newly introduced Pokémon such as Sylveon, Flabébé, and Xerneas, and it was also assigned retroactively to selected older Pokémon, including Clefairy, Jigglypuff, and Marill. This made Fairy both a new category and a way to rebalance existing species.
Fairy-type moves are especially effective against Dragon, Fighting, and Dark Pokémon. Fairy Pokémon are weak to Poison and Steel attacks, while Fairy itself is immune to Dragon-type attacks. These matchups changed competitive battling and made Dragon Pokémon less dominant than they had often been in earlier generations.
Gold and Silver had introduced Dark and Steel types in 1999. No entirely new type appeared in the main series between that generation and X and Y, which is why Fairy’s arrival was considered a major design change.