I remember a story that explained her strange abilities as her being a unicorn, but when she was a foal she fell and hit her horn hard enough to snap it off. She was so young when it happened she never did learn she was a unicorn and not an earth pony.
I wish people would tag their G5 spoilers!
She put ice cream cone on her head like it was a horn and it still emitted magic, at least as seen here.
Think again, Dashie.