When Rainbow Dash accidentally knocked Fluttershy off to defend Fluttershy's honour, Fluttershy fell for hours metaphorically, but instead of butterflies catching her she fall into a tree were vampire fruit bats were sleeping. She was also unfortunate enough to have her neck land on the two fangs of said type of bat.
Two applefarming parents saw this happening in front of their eyes and took her home, the filly transformed into a Pegasus/Fruit Bat hybrid in front of their eyes. It also made the Filly more animalistic with barely anything pony remaining in her. So they decide adopting her in hope they can bring the filly back to her senses (her pony senses that is), and possibly find a cure for her.
But an apple sucking demon for a sister is not really something Applejack would like if she came back from Manehatten.
Also if hadn't caught on that Applejack and McIntosh's parents are around. They are.
Aiming for 0% Twilight (Stephenie Meyer) jokes.
Requested by J-Rook in these two forum posts.
Edit: 03-07-2014 (dd-mm-yyyy)
This story somehow managed to weasel its way onto the popular list. A Firebird first I think... Strange.
Edit: 03-07-2014 And it was at the top for a little while. What the...