I know something about her father breaking it and then she polished it or something. I really don't know. Her name is Vizia, you should google it (she has a blog), some of the stuff there is nsfw, I warned ya
I read it. It's wretched (the situation, I mean). Her father blamed her for his beloved wife's death, so he walked up to her, and smashed her head against the drawer, or something, which busted her horn. She must have sanded it down.
Funny, I had an similar idea for an OC named Silver Cue: he's a Pegasus colt whose father busted his wings up pretty badly, and then had his "Cutie Mark" carved into his flanks with a red hot poker.