Horses are designed for outdoor living and adapt remarkably well to changes in their environment. Each horse is an individual and just like people, each horse has different tolerance levels for cold. In general, most horses that have lived in a cold climate for a while and have an adequate winter coat can tolerate temps down to 20F with no problems. Remember though that if it is snowing, sleeting, or windy then adequate shelter or a blanket may need to be provided. Exceptions to this rule would be thin or debilitated horses; horses suffering from illness/disease; clipped horses; horses recently shipped from warmer climates; show horses. These horses would need blanketing as low as 40F.
Horses create heat from the food they eat and from keeping active. Adequate amounts of good quality hay should be offered. Also, fresh water should be available 24 hours a day, offering warm water would be even better.
The above ideas are very general. Observe your horse outside – is he/she eating, shivering, or playing with their friends? If he/she is standing alone or not eating and shivering then they either need to come inside or be blanketed. If in doubt please talk to your regular veterinarian about recommendations.