From Bulbapedia, the community-driven Pokémon encyclopedia.
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Since Generation II
These Generation II Pokémon appear in the Johto, Hoenn, and Sinnoh regional Pokédices.
Since Generation III
These Generation III Pokémon appear in both the Hoenn and Sinnoh regional Pokédices.
Ralts, Kirlia, Gardevoir, Nosepass, Swablu, Altaria, Duskull, Dusclops, Tropius, Absol, Snorunt, and Glalie were not included in the original Sinnoh Dex as it appeared in Pokémon Diamond and Pearl; however, they all are included (along with their Generation IV evolutions) in the extended version of the Sinnoh Dex in Pokémon Platinum.
Since Generation IV
Until Generation V is unveiled, this list consists of all the Generation IV Pokémon included in the Sinnoh regional Pokédex. This consists of all Generation IV Pokémon excluding only those currently not appearing in any regional Pokédex.