The book on Bettas I have said the long fins of veil tails did occur naturally, but because of this, often didn't survive. Veil tails were eventually bread to enhance the color and length of the finnage, and the other tail types were selectively bred from different traits some fish showed. Some different kinds of domestic bettas were bred from breeding two different types of bettas together.... That is a little bit in a nutshell. I find that kind of stuff interesting and wish someone made a book at least 1000 pages long with info like that :D (pictures are always excellent too ;)