Age and condition of the horse has a lot to do with it. Also the shelter that they have access to. If you have a barn with single stalls then they are out of the wind and rain and can manage on their own....again you would need to factor the condition of the horse. If the horse is old or very malnourished then it can't sustain it's own heat, so blanket them. If your horse is in good condition and at a good weight and he has a shelter (even a lean-to) then he should be ok.