"Shining Dash" by GammaEradon on DeviantArt

Scene from GammaEradon's story "Rainbooms and Royalty" an AU fic where Rainbow Dash is Princess Celestia's student, not Twilight. She crushes hard on the handsome captain of the guard (though she denies it to everypony), who treats her like a second little sister.

http://gammaeradon.deviantart.com