We were talking about how Alicorns could be made. We got to talking about how maybe a genetic mutation could result in the development of one. So yes, we were talking about the show.
All tusks are teeth. A unicorn horn would be a tooth of a pony that just grew too massive and spurt through the head.
That is how you get unicorns, if they existed. However the event of such a thing would pierce the brain of the pony so there will never be unicorns.