As previously teased, today Pokemon Go is getting new Pokemon. For now, only two have been announced, but the game promises to house many more new monsters in the coming months.
Starting today you can hatch Togepi and Pichu, along with several (unnamed) others that were "originally discovered in the Johto Region in Pokémon Gold and Pokémon Silver video games". You visit PokeStops to acquire eggs, and then if you're lucky they will hatch one of the new Pokemons.
Additionally, until December 29 there's a holiday-themed special edition Pikachu which you can encounter all across the world. These Pikachu will be wearing a festive hat, and you should see them appearing in large quantities.