Fluttershy isn't quite a full grown mare yet, but not a filly either. A few days before she moves to Ponyville, she meets somepony named Soarin, who Flutters eventually falls in love with.
Based on my favourite film, "Lady and the Tramp". Cover art from this art on DeviantArt:http://sirsoarin.deviantart.com/art/Soarin-and-Fluttershy-287148599