Niantic has begun adding the very first Generation 2 Pokémon to Pokémon GO, which include baby Pokémon that will all hatch from Eggs that you acquire from visiting PokéStops. Igglybuff and Cleffa can be found in 2km Eggs, while Pichu and Togepi can be hatched in 5km ones. [Thanks, NintendoEverything.]
Now some new baby Pokémon have been discovered to hatch from 10km Eggs, including Elekid, Magby, and Smoochum. Over the next few months, Niantic will be gradually adding more and more Pokémon from the Johto region, but further details have not yet been revealed.
Pokémon GO is available for iOS and Android devices.