I think it's interesting how we just accept differences in animals without specific labels, but yet we do label kids. However, as a teacher, these labels are very important as it helps us know what adjustments we need to make to our classrooms in order to help your child learn best. For example, we know we can't expect an ADHD kid to sit through a lecture, so we need to provide something more engaging to actually get the material across (but then again, who wants to sit through a boring lecture anyway??). Or the kid that has a reading disability may need to hear the text selection out loud rather than be left to get through it in silent reading. So while these may technically be termed "disabilities", for us it's a way for us to identify and get a jump start on a specific set of traits a kid might need us to adapt to.
In the horse world, do we not have similar labels? Horses that are cinchy? Head shy? Don't like to pick up their feet? And isn't it generally accepted that some horses/breeds are more or less intelligent? (I'd argue that Arabs are the "gifted" breed lol ...of course they're also a super emotional breed!) In essence, it's not really necessary to label the horses the way we do kids because we just use more specific descriptions of the issue. In humans, the labels often describe a set of traits that tend to go together.