Niantic announced that new pokemons available in the Pokemon GO in all regions. With the latest update two popular ones available: Pichu and Togepi.
According to the press release of the US version of the game several other ones will be available from the Johto Region from the original Pokémon Gold and Pokémon Silver games.

From December 29, 2016 a special Holiday event going to starts where you can find a rare, Santa edition Pikachu! More information about this event coming soon! Now go and catch the new pokemons.