Which Pokémon generation introduced the Fairy type?

Generation VI introduced the Fairy type. The type debuted in Pokémon X and Y, released for the Nintendo 3DS in 2013.

Fairy was the first new Pokémon type added since the introduction of the Dark and Steel types in Generation II. Its addition changed type matchups, especially by giving Fairy-type attacks an advantage against Dragon-type Pokémon.

Some older Pokémon were retroactively assigned the Fairy type. Jigglypuff, Marill, and Gardevoir are examples whose classifications changed in Generation VI, so not every Fairy-type Pokémon was newly introduced in X and Y.

Generation VI ultimately used all 18 types recognized in the modern main series. The games also introduced Mega Evolution and moved the core series to fully polygonal 3D environments.