When Rarity is manipulated by the nightmare forces into making her believe others would abandon her, she transforms into Nightmare Rarity, simmilar to Nightmare Moon. When she is in this form, she speaks as though Rarity is a completely different pony, and that she is the same as nightmare moon, saying that ponies had defeated her before.

She doesn't appear in the show, but does in the IDW comics.

Much like what happened to Luna during her time as Nightmare Moon, Nightmare Rarity is much taller and has a darker coat. She also gains a diamond on her horn, a longer mane and tail and a cutie mark more in the shape of a star.