From what I've seen, I think this girl is someone an Outcast...she's in the Pony Posse, but doesn't talk or hang with them, they don't hang or talk to her. There is something up with her. Maybe she's mean because the group they have set her up with won't really have much to do with her. She ALWAYS has a look on her face as if she hates being there...which maybe her parents are forcing her to ride? She looks about 12.
I think I may try to talk nicely to her and find out what's up. If I'm right, then maybe I can be someone she can talk to, if I'm wrong and she's just a big fat B and that's why the girls don't like her than I can then go to management.