It was just a genetic mutation in the breed. The Indians considered them sacred just like they did in some tribes with the Appaloosas. But then some people began to slaughter them when they were starting to be sold on the market because they were "unusual". It drives me NUTS when people think that their coats are hard to care for. I think that they are the most easy to care for horses as far as coats go! Some curlies have the really fine crushed velvet feeling coat and then others have a slick smooth coat with just the curly mane and tail. And their coat in the winter repels ice and sleet and whatever else. They look like buffalo sometimes. My one mare does in the winter she gets so curly. Lol Its hilarious!