I have a question, I have heard on some of these other threads, that horses have 'straight shoulders' or 'rounded shoulders'... Just wondering what the difference is and if there are other types of shoulders, and how it effects the horses movement/ performance. Pictures and diagrams would be lovely!
(And sorry if its a dumb question, I was just never taught this, and want to learn more about it!)
The shoulder blade, measured from the top of the withers to the point of shoulder, lies in an upright position, particularly as it follows the scapular spine. Often accompanies low withers.
Upright shoulders are common and seen in any breed, particularly Quarter Horses. An upright shoulder affects all sports.
The horse has shorter muscular attachments that thus have less ability to contract and lengthen. This shortens the stride length, which requires the horse to take more steps to cover ground, and thus causes a greater risk of injury to structures of front legs and hastened muscular fatigue.
An upright shoulder may cause a rough, inelastic ride due to the high knee action. It increases concussion on front limbs, possibly promoting the development of DJD or navicular disease in hard-working horses. The stress of impact tends to stiffen the muscles of the shoulder, making the horse less supple with a reduced range of motion needed for long stride reach.
An upright shoulder causes the shoulder joint to be open and set low over a short, steep arm bone, making it difficult for a horse to elevate its shoulders and fold its angles tightly, which is needed for good jumping, or in cutting. A horse with an upright shoulder usually does not have good form over fences.
The horse is usually easier to accelerate in sprinting.
An upright shoulder is best for gaited or park showing, parade horses, and activities requiring a quick burst of speed, like roping or Quarter Horse racing.
Laid-back or sloping shoulder
The horse has an oblique angle of shoulder (measured from the top of the withers to the point of shoulder) with the withers set well behind the elbow. Often accompanies a deep chest and high withers.
A sloping shoulder is common. It mostly affects jumping, racing, cutting, reining, polo, eventing, and dressage.
The horse has a long shoulder blade to which attached muscles effectively contract and so increase the extension and efficiency of stride. It distributes muscular attachments of the shoulder to the body over a large area, decreasing jar and preventing stiffening of the shoulders with impact. The horse has an elasticity and free swing of its shoulder, enabling extension of stride that is needed in dressage and jumping. A long stride contributes to stamina and assists in maintaining speed.
The longer the bones of the shoulder blade and arm, the easier it is to fold legs and tuck over fences. The laid back scapula slides back to the horizontal as the horse lifts its front legs, increasing the horse's scope over fences. 
A sloping shoulder has better shock-absorption and provides a comfortable ride because it sets the withers back, so a rider is not over the front legs.
A sloping shoulder is most advantageous for jumping, dressage, eventing, cutting, polo, driving, racing, and endurance.