When is the winter temperature too cold for a horse to stay outside?

If the horse is acclimated to being outside and has an opportunity to grow a winter coat, the horse will be O.K. all winter. If you are going to house a horse outside, here are a couple of things to consider:
1) Make sure the horse has adequate water available. Horses may become dehydrated, even in the cold weather, if the water is frozen most of the time. The horse will not consume enough snow to meet its water requirement.
2) If you are going to blanket the horse, make sure that the blanket is intended for a horse living outside. The outer surface of the blanket will be made of a more durable material, usually a canvas-like fabric, that will stand up better to being outside than the quilted blankets used on stalled horses. Also make sure the blanket is checked regularly for any damage that may create a place for the horse to get caught in the blanket. If the horse is acclimated and has shelter, it can be comfortable without a blanket even when living outdoors.
We do not recommend hooding a horse that is living outdoors. There is too much opportunity for issues to arise such as the horse to get the hood caught on something and become injured trying to get free.
Finally, provide the horse with plenty of hay throughout the winter. Digesting the forage produces heat, which assists the horse in maintaining its body temperature.