In Becoming Twilight a series of events causes Twilight Sparkle to separate a part of herself into an attacking stone pony and that part of herself bonds with it and becomes a separate creature all on her own. This is what happens to that stone pony afterwards.
Art by JustThefangirl: http://missanimegrl.deviantart.com/
Twilight has recently become an alicorn. But being reborn isn't all it's cracked up to be...
Alabaster used to be a part of one of the most powerful ponies in Equestria. Now she's a pony made of stone learning to live without magick.