Erin wasn't at all happy about being left behind. The Royal Guards had gone through, along with Twilight and her other friends, as well as Celestia herself. However, she'd been told that she wasn't allowed to go along. It made sense, in that she wouldn't be able to help in any significant fashion, but it just bothered her that her friends were facing the premier threat to her own world and she couldn't even stand by them. 