Inside the stall, the massive horse balanced precariously on three legs, supported by a sling around his 1,500-pound body that was suspended from a one-ton hoist in the ceiling. His name was Piaff and he was Dutch Warmblood gelding the color of dark chocolate. His left hind leg was too close to the right and knuckled over at the fetlock. Dr. Stephen Reed stood at Piaff's head while two interns...