No, Outlast does not work on horses. Outlast is a fabric technology that was specifically designed to keep humans comfortable in extreme temperatures. It works by absorbing and releasing body heat as needed so that the wearer remains at a more constant temperature throughout the day. Its effectiveness relies on insulation between the skin and the material; this kind of insulation isn't present when it comes to horses because their coats are designed to do just that - insulate them from outside temperatures. Therefore, using Outlast fabric for horse clothing would be ineffective and provide no additional benefit.