What is the set with a black R like Team Rocket, but has a red background. I forgot the name of it and can’t find it online. Thanks.
Team Rocket Returns would be my main guess from your description, but the Red “R” featured on these cards is not the set symbol, but a distinction given to “Rocket’s Pokemon” in the set.
The set symbol for Rocket Returns is: 
As featured on cards such as in the bottom right hand corner: 
There is a distinction between “Dark” and “Rocket” Pokemon in the set! The Dark Pokemon are not “Rocket” Pokemon, so they do not receive the red R mark. All “Rocket” Pokemon AND Rocket related trainer cards are marked with the red R. Examples:
Rocket Pokemon also appear as secret rares in EX Deoxys and EX Unseen Forces as these two cards:
So, as you can see, the red R is not exclusive to the Team Rocket Returns Set. It’s a symbol to indicate a Pokemon being owned by a certain faction/trainer.
A similar marking can also be seen on cards during the Platinum era expansions, where all Pokemon cards owned by trainers/ a faction are given an “SP” on the left hand side just below the image of the Pokemon.
Hope this helped! 
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EX Team Rocket Returns?
Everytime I forget a set I just go on the bulbapedia page. bulbapedia.bulbagarden.net/wiki/List_of_Pok%C3%A9mon_Trading_Card_Game_expansions
NVM I found it. It was a coro coro card. Coro Coro Meowth. Not sure why they printed it different.