Pokemon Go recently added two new kinds of Pokemon as part of its new Team Rocket event. Specifically, this began when Team Rocket started its takeover of PokeStops in Pokemon Go. As a reward for driving away grunts from various PokeStops by battling them, players have a chance to capture Shadow Pokemon, which are Pokemon used by Team Rocket in their nefarious schemes. You can tell Shadow Pokemon apart from their normal iterations because the Shadow ones are covered in a dark energy and have deep red eyes, and they also feature some serious drawbacks. They're much harder to level, with double the Stardust and Candies cost, and they're saddled with the weak Charge Move Frustration.
Once a player catches a Shadow Pokemon, they have the option of Purifying it by spending Stardust and Candies. Not only does Purifying a Pokemon remove the negative attributes a Shadow Pokemon has, it also gives the Pokemon a permanent +2 boost to their IV stats and automatically levels the Pokemon up to Level 25. Leveling up a Purified Pokemon also costs fewer Candies and Stardust, even after they evolve. A Purified Pokemon also automatically learns the Charge Move Return, which is better than Frustration.
While there could be some surprises still waiting in the, pardon the pun, shadows, confirmed Shadow Pokemon include Bulbasaur, Ivysaur, Venusaur, Charmander, Charmeleon, Charizard, Squirtle, Wartortle, Blastoise, Rattata, Raticate, Zubat, Golbat, Crobat, Snorlax, Dratini, Dragonair, Dragonite, Mudkip, Marshtomp, and Swampert.
Team Rocket appears to be the next step in Pokemon Go's ongoing evolution, and could introduce some much needed story and new content to the game or at least serve as an introduction to more.