Well of course that isn´t practical. Do you, therefore, enjoy your chimney or central heating whilst riddled with guilt about your naked equine out in the "cold"? Of course not, and here is an excellent article explaining why.
http://mhwf.websitetoolbox.com/post/equine-thermoregulation-in-cold-weather-6697515
You must consider the article focus´s on horses being correctly maintained, naturally. Horses that have forage available 24/7 (just as it should be) and are not filled on 2 or 3 meals of highly processed feeds that consist of short fiber´s, meaning they are processed very quickly through the gastrointestinal tract, unable to provide any natural internal warmth as they pass on their speedy way. This also does not take into consideration horses with PPID o