For all stories where one of the Mane Six, a canon background pony (or creature), or even an alicorn princess is actually a changeling infiltrator or abandoned changeling adopted into a pony family.

Note that this is a group for stories where a pony we know and love is actually a changeling, such that we see the struggles they endure being a changeling in pony society — either undercover, being uncovered as a changeling, or never knowing until a self-image shattering reveal. This is not a repository where a character we know is replaced by a changeling, or if the pony is transformed into a changeling during the course of the main story.

For a prototypical example, review The Three Sisters and sequels.

(I don't know yet whether I'll allow OCs (Original Changelings) into this group. I'll decide later.)