I read a book called "Ride the Right Horse: Understanding the Core Equine Personalities and How to Work with Them"
by Yvonne Barteau. http://www.amazon.ca/Ride-Right-Hors...3173431&sr=1-1
According to the book:
Social horses are the ones always staring over the stall door that have to be a part of everything. They are one of the easier horses to train, but are easily distracted while being ridden. They bond with many individuals, whether human or horse.
Aloof horses are the "anti-socials", and often stand in the back of their stalls. They rarely bond with other horses and humans, and are usually by themselves in the pasture. They tend to tune things out/ignore aids while being ridden.
Fearful horses are just that; they are startled easily and look to others for support. They like a constant schedule and require a patient, quiet rider. They tend to bond strongly with only one or two people/horses.
Challenging horses love to be on top, and are always pushing the boundaries. The key thing with them is to make sure that they respect you. Otherwise they will find any and every way to take advantage of the inexperienced/fearful rider and exploit the rider's weaknesses. However, they're very competitive and excel in many disciplines with the right handler.
There can also be different combinations of personalities, like aloof-fearful, social-challenging, challenging-aloof, etc. etc.
It was a great read, and it really made me think. It definitely makes sense about how different horses bond with people, while others don't.