Posey is a strange fusion of Applejack and Fluttershy created by Innocent Sun's subconscious mind in hopes to comfort herself. She's got Flutters' kindness, and Applejack's closeness with Sun. When Sun started getting nightmares which progressively brought her closer and closer to remembering the death of her parents that she forced herself to forget, Posey was manifested as a sort of guide for her to accompany her through the nightmare-recreations of her hometown, Pony Creek. Although for the most part they managed to evade the attacks of a monster always unseen but always present, Posey was injured by it just as Sun found the strength to face reality. The mare was shocked when she found that her injuries revealed that the inside of her body was entirely hollow. It was then that things suddenly started to not make sense… How come she was apparently childhood friends with Sun even though she couldn't remember anything? How did she even know what she did about Sun? Why did even being herself feel weirdly… off? Like she wasn't meant to exist?
Well, she tried to find Sun for the answers, just as the unicorn was ready to leave Posey behind. In fact, that was what Sun told her: Posey was only created to provide her comfort, and now that she could face reality — that her parents were dead — she didn't need Posey anymore. Posey admitted she knew that deep down, but she genuinely wanted Sun to succeed, so she didn't care. In fact, she was happy that Sun no longer needed her. After saying goodbye, Posey smiled at Sun as the former disappeared into the deepest recesses of the unicorn's mind, and the former left her nightmare behind, before finally facing reality.
Don't ask why she's dressed like a bootleg Heather Mason.