This MLP papercraft is the Nightmare Moon, designed by muffinshire. Nightmare Moon, also known as the Mare in the Moon, is an Alicorn pony first seen with blue armor and magic mist hair, and is the main antagonist of the first two episodes. A thousand years ago she was imprisoned in the moon by Princess Celestia, until finally her prison breaks and she is once again freed, whereupon she uses her strong magic to capture Celestia and prevent the sun from rising, to make Equestria a land of eternal night.
In an attempt to stop Twilight Sparkle and her friends from reaching the Elements of Harmony, the powerful artifacts that Celestia used to defeat her before, Nightmare Moon uses her magic powers to put a series of dangerous threats in their path to hurt, threaten, or lure them off their quest. At the end of "Friendship is Magic part 2" she is revealed as Princess Luna, Princess Celestia's little sister who is very remorseful of her own actions.
You can download the paper toy here: My Little Pony - Nightmare Moon Final Prototype Free Paper Craft Download