Horses have an amazing ability to thermo regulate. That means they have natural mechanisms that allow them to stay warm and dry when it is cold, raining or snowing, and cool off when it is hot. Many horse owners blanket their horses year round because they are trying to keep the hair coat short and the horses relatively clean for shows. If you do not fall into this category rest assure with proper feeding and a good shelter where they can get out of the wind and rain your horse will be quite comfortable this winter without blanketing. However if you must blanket your horse because the guilt is absolutely killing you I would recommend a light weight blanket so that the horse does not over heat.
For some horses it is good to blanket them with a water proof blanket when it is raining. However if the blanket is not waterproof, and it gets saturated with water, this can be more detrimental to your horse because they will not be able to get dry and will get chilled from the wet blanket. Please read the following article about the horse’s incredible natural ability to thermo regulate and all of the different mechanisms it can use to either warm itself up or cool itself down. It will only reinforce to you how truly amazing these incredible animals are.