The [Unicorn](https://en.wikipedia.org/wiki/Unicorn) is a iconic symbol of fantasy, appearing in both mythology and modern novels. It is simply a horse with a single, spiral horn erecting from atop its head. How could a creature like this realistically evolve? How close to the stereotypical unicorn can I get using realistic anatomy?

A list of all of the Anatomically Correct questions can be found here 

[Anatomically Correct Series](http://meta.worldbuilding.stackexchange.com/questions/2797/anatomically-correct-series/2798#2798)