Horses have often served as a model for understanding evolutionary change as their transitional forms from small species with multiple toes to larger ones with a single digit are fairly well documented. What is less often reviewed is the unique structures of their upper respiratory system...primarily the guttural pouches. These pockets are unique to horses (and rhinoceros) and present an alternative evolutionary design for cooling one of the most important organs in the body, the brain. Many species use counter-current circulation to accomplish this, but the guttural pouch achieves the same goal with air. A network of sinuses and a well developed hyoid apparatus complete this unique system within the equine head.