Horses do fine outside as long as they have a lean to shed. You have to remember that wild horses survive in much harsher conditions and as long as you provide some extra hay and grain in the winter, and a windbreak, such as lean to, they will do fine outside. They will grow longer furry haircoats and it really shouldn't be a problem to keep them outside even in the winter and you can bring them in if the weathr gets really extra bad.