Twilight has a new spell that's almost ready to show to the princesses, one designed to grant mares a package fit for a stallion. Only problem is all non-battle spells need to be tested on foals first to determine their safety and effectiveness. Luckily Ponyville has three little fillies who are all too eager to try new things!

Note: I haven't been keeping up with the show since S4 so for the sake of not wanting to portray things too inaccurately (even if this is self-indulgent smut) this takes place around season 2, pre-Alicorn Twilight and pre-Cutie-Marked Crusaders.

CONTENT WARNINGS
All chapters contain, to at least some degree: Foalcon, futanari, transformation, and casual nudity (like the show, ponies only wear anything more than a hat on special occassions, but unlike the show they've got genitals)
Scootaloo's chapter focuses on the transformation itself and her building orgasm and contains musk and an accidental facial
Apple Bloom's chapter focuses on her giant schlong and excessive precum and contains penis awe, precum leaking and collection, and the closest thing to sex this story has (a hindhoofjob, autofellatio, and frotting as they explore their new anatomy)
Sweetie Belle's chapter focuses on her internal thoughts and mild humiliation and contains reluctance, light SPH (small penis humiliation), and a continuation of the playful exploration from the previous chapter