Generation II began the Pokémon series's expansion-focused nature, introducing to the world 100 new Pokémon which did not exist and are unable to be obtained in the Generation I games and the new region of Johto. Many of these Pokémon expand the evolution families of older Pokémon, while a majority of them are brand-new evolutionary families.

The initial hint that Generation II was on its way came in early 1997, with the release of the anime's first episode. A Pokémon appears to Ash Ketchum on the first day of his journey, shortly after he and Pikachu become friends, that cannot be identified by the Pokédex. This magnificent golden bird, later revealed to be the legendary mascot of Gold Version, Ho-Oh, was the first Pokémon from a future generation to debut in the anime.