Pokémon GO just announced a few new exciting updates! Pokémon GO just released Ditto a few weeks ago and now Togepi and Pichu will be available starting later today. You’ll be able to get these Pokémon – which are from the Pokémon Gold and Pokémon Silver games – by hatching eggs.
— Pokémon GO (@PokemonGoApp) December 12, 2016
Also, they’ve announced on their blog that starting later today and lasting through December 29, “limited edition versions of Pikachu will be available”, and “he’ll be wearing a festive hat to celebrate the season.”
Source: Pokémon GO