A non-cancerous mutated mole of some sort... weird stuff. In more developed countries, doctors would have snick that off at the first sign of oddity so it'd never get to that size. Moles that change shape can be very bad news.
Quote Originally Posted by the original poster
I wanna grow a horn!
Some body-modifiers have had horns installed - either synthetic horns that sit under the skin (though there's a risk of the skin on top dying), and some who go for the more extreme measure of having tiny bumps of live coral attached to the surface of their skull. The coral then keeps growing, so the horns gradually grow over time.