A Unicorn is a type of pony that has magical powers and a horn on the top of their head. While they don't have wings, it is possible to achieve flight, as seen by the butterfly spell that Twilight Sparkle puts on Rarity so they could cheer Rainbow Dash on at a flight competition. While all unicorns can be seen doing basic magic, mostly involving levitating objects, Twilight Sparkle seems to be capable of far more powerful and different magic. Some notable unicorns include: Twilight Sparkle, Rarity, Starswirl the Bearded, and The Great and Powerful Trixie.
Related ThreadsGB's Guide to Pony Fanon! - last post by @ Aug 12, 2012
[My Weekly Pony] Ep. 6 - Boast Busters - last post by @ Dec 3, 2014
[My Weekly Pony] Ep. 7 - Dragonshy - last post by @ Dec 4, 2014
[My Weekly Pony] Ep. 1 - Mare in the Moon - last post by @ Dec 3, 2014
My Little Pony: Friendship is Magic - Fanfiction Contest! - RESULTS ON PAGE 7 - last post by Bread Effect @ Nov 10, 2011