Every action has an equal and opposite reaction, and so when Princess Cadance gives birth to her firstborn foal, Skyla, another princess, Asher, also emerges. But Asher hails from the Changeling Kingdom and as such is used solely as a replacement while her fellow changelings kidnap the real princess. Asher goes about her life acting as a princess, and once she becomes a filly, she begins to realize that she was never meant to be a changeling. As a side effect of having a mother who switched from a pony to a changeling, Asher herself swings the other way around, wishing she could have Cadance and Shining as her real parents and that the scheme never happened. But before long, the royal couple finds out, and chaos might just ensue...

Sorry if this may have been done before, but I found out about a Changeling Princess only after writing this. So sue me. xD