I usually dont do this, but since Im against the alicorn-OC-bashing (or drama), I googled a bit for this alicorn OC to write a description for this pony:
 
The alicorn OC is called Shiron and has his own asktumblr http://askshiron.tumblr.com/ .
 
Usually this OC is a pegasus pony but for a fanfic called Rainbow in the sky, Shiron had to be an alicorn with a different cutie mark and different background story. The fanfic is on dA and fimfiction and the author only made an alicorn OC to show that being an alicorn is a curse and a gift. So, although he might have the potential of becoming very powerful, the main reason seems to be to show not only the good parts (like power) but also the bad parts (immortality or that others treat alicorns different and also fear them sometimes like Twilight Sparkle, who always think that Celestia might react in a bad way).
 
 
I read that fanfic but the author, Shiron or Shriek, sadly takes long to update new chapters. Although the author and the OC are having the same name, it isnt self-insert. The author answered that hes bad with coming up with new names. http://www.fimfiction.net/story/1170/rainbow-in-the-sky