It is amazing how resilient horses are. They are naturally programmed to deal much better with subarctic temperatures then heat. Some of us feel bad when we sit in front of the fire watching our horses through the  frosty windows. However don’t despair – horses will  choose to stay outside all around the clock rather than being stalled up. If your horse is generally in a good health, isn’t clipped,  has an access to a run in a shed and free choice hay, he will greatly benefit from staying outside at all times. Healthy respiratory system,  better fitness, prevention of joint stiffness, decreased occurrence of ulcers etc. If your horse is clipped, has a thinner coat or doesn’t have enough “natural insulation” , go ahead and use a blanket.  Make sure it fits properly though. Ill-fitting blankets is are very uncomfortable – just ask your horse.