About the Sacroiliac Joint
It is not unusual for horses to develop SI pain as this joint handles tremendous force every day. "The sacroiliac joint is the intersection of sacrum, the section of the spine that underlies the croup, and the ilium, the largest of the bones that make up the pelvis. The sacrum is made up of five vertebrae fused solidly together to form a single unit. The ilium is shaped like a fat T, with a narrow shaft that flares out into a wide, flat crest. In conformation diagrams, the spot known as the point of the hip marks the outer branch of the crest, called the tuber coxae. (The true hip is actually farther down, at the base of the ilium shaft.) The inner branch (tuber sacrale) ends over the sacrum, at the top of the croup. This is the part of the bone involved in the SI joint. If you viewed a cross-section of the joint..., the two inner branches (one from the bone on the horse's right side, one from the left) would form an arch, with the sacrum in the cleft between them. Inside the joint, smooth cartilage covers the working surfaces of the bones. A close-fitting membrane encloses the joint and secretes lubricating fluid. Broad, strong ligaments [-] the dorsal (upper) and ventral (lower) SI ligaments [-] lash the bones together tightly. There's very little movement in the SI joint; it's designed for shock absorption and stability, not mobility. It has to be strong to hold up under the force of the horse's movement." Sacroiliac Joint A Closer Look
Dr. van Wessum's six indicators of SI disease: The DIY Test for SI Disease
This link is a comprehensive article on sacroiliac disease in horses.