This is Applejack's sister Apple Bloom in the outfit she wore in the battle of the bands in Equestria Girls: Rainbow Rocks sequence. She and other members of Cutie Mark Crusaders, CMC, appear as a group. It was a such cute outfit that I couldn't resist it.

Apple Bloom is a character in My Little Pony: Equestria Girls. Like many of the other characters in this movie, Apple Bloom has a pony counterparts in Equestria.

Apple Bloom appears in Equestria Girls for the first time with Sweetie Belle and Scootaloo when Twilight Sparkle is singing This Strange New World (song) next time she is seen is when she is in the library with Scootaloo and Sweetie Belle (humanized version of the CMC). Apple Bloom is seen in the Equestria Girls (Movie Song), as well as the song This is Our Big Night.
She is around the age of the younger ponies/humans, so she it is thought that she is a high school freshman in the movie. Even though she is in Elementary school in the show.
Human Apple Bloom wears her signature pink ribbon in her hair, a green shirt, jeans and orange galoshes. At the Fall Formal, she is wearing a sleeveless lace-trimmed lavender-coloured dress with a light violet ribbon around the waist. There is a flower in the middle of the ribbon, which might be a reference to her name (Apple Bloom.)
Apple Bloom and her fellow Crusaders appear in My Little Pony Equestria Girls: Rainbow Rocks.
They first appear in the gymnasium working on their poster for the upcoming Canterlot High School Musical Showcase. When the now-reformed Sunset Shimmer asks if she can help them, Apple Bloom politely declines the offer, still haunted by the Fall Formal events.