And this is why I love this thread. For awesome random facts of win.
Plus looking at the alternatives the rest just sound so bad. Alicorn sounds like a real word (being a real word and all ) as well as sounds appropriately regal. However I would find it highly amusing if the ponies of Equestria don't actually have a name for the Princess' species and we get a scene where ponies argue about the correct name for the species.