Through the Mirror (My Little Pony Equestria Girls, Bk. 1)
When her new crown is stolen from the Crystal Empire, Princess Twilight Sparkle pursues sunset Shimmer into an alternate world, where she finds herself turned into . . . a girl!
Twilight must find her place in this strange new world and defeat Sunset Shimmer in a competition to become the Canterlot High Fall Formal princess. Luckily, she has the assistance of Spike and five girls who remind her of a certain group of friends back home. . . .