Well, the horsenality concept is a large part of the horse personality topic as such - many use it, many find it at least interesting, many have learned from it as beginners and, if somebody got to read something about the horsenalities without knowing it comes from PP - I bet he'd find it useful or relate it to horses he knows. ;)
Another kind of horse personality list is done by Klaus Ferdinand Hempfling (my personal inspiration), 26 types of horses, grouped both by their characters and by physical features that can tell us about their minds. However, the system is very vast and complex, and not intended to be used as a simple test or a superficial generalisation. It is fascinating to look into, though. You can find about it on Youtube and in his site.
I have come a long way, to surrender my shadow to the shadow of my horse.