This is Zipp Storm. When I originally saw the character's design I wasn't to thrilled. However she ended being the surprise character for me in the film, and the one I want to learn more about her and her interest piecing together what actually happened long ago. She might end up being a useful in Sunny's own quest for the answer.
One design feature that caught my attention was that the ponies only had a cutie mark on their right side. I'm curious to see if a possible upcoming series will address that issue.