The official Pokémon website has announced that new Shadow Pokémon have been added to Pokémon GO like Vulpix, Ninetales, Exeggcute, Exeggutor, Omanyte, Omastar, Misdreavus, Carvanha, and Sharpedo.

New Shadow Pokémon—Like Vulpix, Raikou, Mawile, and Beldum—Join Pokémon GO

Team GO Rocket may have captured a whole new batch of Shadow Pokémon, but they didn’t count on you coming to the rescue.

Team GO Rocket have captured more Shadow Pokémon in Pokémon GO, and it's up to you to stop them, Trainers! When you challenge Team GO Rocket Grunts at discolored PokéStops, there's a chance they'll have Shadow Pokémon like Vulpix, Ninetales, Exeggcute, Exeggutor, Omanyte, Omastar, Misdreavus, Carvanha, and Sharpedo on their team. Team GO Rocket Leaders might have Pinsir, Mawile, and Beldum. And if you're up to the challenge, face off against Giovanni and the Legendary Shadow Pokémon Raikou!

Once you defeat the villains, you'll have the chance to catch one of their abandoned Shadow Pokémon. You can then choose to purify the poor Shadow Pokémon after you add it to your team.

It's up to you to save these neglected Pokémon, Trainers. Good luck!

Source: https://www.pokemon.com/us/pokemon-news/new-shadow-pokemon-like-vulpix-raikou-mawile-and-beldum-join-pokemon-go/