Horses are similar to many other species with the skin being the largest organ in the body. This is based on overall organ size; just think about the total surface area of a 600 kg horse!; The good news is that it can be easily evaluated by visual inspection and palpation. Specific tasks for equine skin include protection from trauma, thermoregulation (temperature control; sweating and heat conservation), sensory perception, secretory function and pigmentation. In addition, it is important to recognize that skin is an important indicator of systemic health since disorders of the skin may actually indicate systemic ill health.
Elizabeth G. Davis, DVM, PhD, DACVIM, Kansas State University (16 October 2006)