Currently, all Generation I Pokémon are available in Pokémon GO, except Legendary and Mythical Pokémon, but no Pokémon from other generations. Other than the recent addition of Ditto, this will mark the first time new Pokémon have been added to the game since its launch in July this year.
Press release
“The Pokémon Company International and Niantic, Inc. will reveal details on December 12 about the addition of more Pokémon to Pokémon GO, one of the most popular mobile games of the year. Stay tuned, Trainers!”