Flutershy rewrote Erccre147's arrangement of This Day Aria for the flute. I don't play the flute myself, but I'm sure the song sounds great on it, the tonality seems to fit perfectly.
You can find Flutershy's arrangement here (on Noteflight). It's a great resource for any flute players out there, so take a look! Many thanks to Flutershy for sharing her musical talents.
(Side note: This image has a hilarious animation mistake. It makes me giggle, although I'm afraid Evil Cadance will come after me for laughing at her.)