Cartoon Network / The Pokemon Company
References to 100 new creatures (ranging from number 152 to 251 in the master Pokédex) are already hidden in the game's code, and it's only matter of time before they're unleashed into the world.
Certain Pokémon that are currently in the game, like Eevee and Seadra, will be able evolve into unreleased second-gen Pokémon, so you'll want to stockpile their candies now to prepare.
We've used data complied by the Pokémon Go experts from The Silph Road on Reddit, Bulbapedia, and official creature descriptions from the master Pokédex to put together the following list: