Pokémon GO has made Generation II Pokémons from Pokémon Gold Version, Pokémon Silver Version and Pokémon Crystal Version available today (13th December). Not all Pokémons from Generation II are released today, however.
The only certain way to acquire the latest collection is through egg hatching, but Pokémon GO said there could be other ways to get the Pokémons.
A Christmas Pikachu is available starting from today to 29th December.