After the Nightmare Forces convince Rarity that while others might eventually abandon her for someone better and the Forces would always want her help, Rarity is manipulated into becoming their queen.
Just like Princess Luna's Nightmare Moon persona changes her appearance, Rarity's height is increased and she gains a darker coat, a diamond on her horn and a much longer mane and tail. Even her cutie mark changes to become more star-like.
In Issue 7, Nightmare Rarity speaks of Rarity as if they are two separate characters by saying Rarity was her specific choice as a vessel.
Out of Character info
((It's good to be back, I am open for the most part though work may interrupt during the afternoon.))
Some of this page is derived from the Friendship is Magic Wiki.The original article is at Rarity#Nightmare_Rarity.