In Pokémon GO there are now 11 new Shadow Pokémon that you can obtain in battles with Team Rocket. Here is an overview.
What is new? After the Rocket Grunts started to invade PokéStops a few days ago, they mainly battled with Rattata, Zubat, and the starters. Occasionally with Snorlax or Dratini.
But now there is a new wave of Shadow Pokémon that Team Rocket uses in battles. This includes Magikarp, which can evolve into the fan-favorite Gyarados.

List of all new Shadow Pokémon
These Pokémon are now also Shadow: A total of 11 new monsters can now be used by Team Rocket in battles. Some Pokémon can still evolve, but Team Rocket does not battle with these evolved forms. Here is the overview:
- Quapsel
- Sleima
- Sleimok
- Traumato
- Hypno
- Tragosso
- Sichlor
- Karpador
- Hunduster
- Hundemon
- Trasla (not yet in the game)

Which Pokémon are worth it? Strong counters are the evolutions of Scyther and Magikarp. It is definitely worth hoping for Pokémon with good IVs. Scizor is one of the best Bug attackers in the game and Gyarados is a solid Water attacker.
The other Pokémon do not play a significant role in the meta of Pokémon GO. Only Trasla could become interesting again, but this Pokémon is not yet available.
This is what has been said about attacks: Niantic explains that exclusive attacks from Community Day can be learned by redeemed Pokémon. Shadow Pokémon, however, cannot learn such attacks.
If Trasla comes to the game before its Community Day, you should redeem and evolve it. Then it learns Synchronoise.
Are there Shinys of this? No trainer has reported an iconic Shadow Pokémon yet. We can therefore assume that the Shiny version of the Shadows is not yet in the game.
Redeeming Shadow Pokémon has now become cheaper or more expensive: