Twilight cast a spell and got herself turned into a filly. Luna has tricked Celestia into believing there is no cure, and now as Luna tries to 'find a cure' Celestia has to look after Twilight the Alicorn; filly version. And heaven knows Alicorns are not born often for a reason.
Traducción Española: https://www.deviantart.com/spaniard-kiwi/art/Potras-alicornio-de-alto-mantenimiento-y-tu-990270118