There is no such thing as the perfect horse. However, how the pieces of a horse “fit together” can significantly influence how that horse performs and help that horse stay sound throughout its career. For any discipline the overall balance of the horse is important.
However, when you start looking at different disciplines, you look for horses with conformational attributes that will make their job easier. For
example a horse that excels at Western Pleasure is likely to look different to a top level dressage horse or a jumper. The western horse will likely have his head and neck set lower onto his body than a dressage horse, who will have a more upright
conformation and will likely have a shorter neck that ties in higher. A jumper, or event horse, will likely have a longer neck (as that is useful for balance over fences).
One of the best resources that I’ve found on understanding conformation is Judy Wardrope’s, JW Equine. She has several articles that highlight the conformation attributes that most suit specific disciplines. The links to the articles below are PDF files.
Conformation 101: What do you look for in a Jumper?
Conformation 101: What to look for in a Dressage Horse?
Conformation: Hunter or Jumper?
I also found this video from eventer Phyllis Dawson to be helpful.
I really like that JW Equine page. She and Deb Bennett agree that the LS joint is the most important part of the horse’s body. If that is good it can make up for a lot of other faults.
I would like to see more people look at the conformation of a horse before deciding what direction to take it in. I worked on a “reining” horse last week who had such muscle spasms going on in her top line she couldn’t even walk comfortably. I put reining in quotes because her conformation was so wrong for this sport. Poor conformation for their chosen discipline is a common reason for people seeking massage therapy for their horses. They just don’t know that’s why their horse is in pain.