From Bulbapedia, the community-driven Pokémon encyclopedia.
This is a list of Pokémon by the anime episode they debuted in. Please note that Bulbapedia considers the movies to be separate from the main anime, regardless of their canonicity, and therefore Pokémon like Giratina, Latios, and Latias, which have not yet appeared in an episode, do not have an episode listed.
This list counts only actual appearances as official. Pokémon which appear on TV in the anime or in another manner before debuting in physical form, such as Nidorino and Gengar in EP001, count as debuting when they physically appear, not when they are first seen.
List of Pokémon by anime debut