Background pony from the movie. She roughly resembles Ballad

or Lilac Notes

Alas, we can't see her cutie mark (unless someone manages to bring along some Blu-Ray screencaps, but even then she's way too much of a background pony), nor can I find any screencaps of Ballad or Lilac Notes showing their eye colours. Besides, her hair seems a bit too light in comparison.

If you think she's an entirely new pony, you may suggest names in the Background Pony Naming Thread. For now you can find her under not ballad.