I'm going to caveat what I'm about to say. If you live in a part of the world that horses cannot exist in nature (that would be the very, very northern area of Canada, Alaska, Russia, etc.... and Antarctica) and you have a horse you can ignore part of what I'm saying, but keep the rest in mind because it applies no matter where you keep a horse.
Rugging or blanketing a horse is something dreamed by humans that was not a good thing when they came up with it and, like the European practices of bad diet and shoeing, did not suddenly become a good idea just because they did it for a long time.
Only humans would have the arrogance and unmitigated gall to think that we know what's best. After all, we've been dealing with horses (for something other than a menu item) for over 5,000 years unlike Mother Nature who's only been evolving and taking care of them for only.....10,000,000 years. Clearly humans have the upper hand over nature in knowing what's best.
First. How nature designed the horse and why it survived successfully over so much of the world and why humans were able to use keep horses in about 80 of the areas on earth that we inhabited (LONG before someone came up with the idea of wrapping them up for cold weather). Unless people interfere (keep that in mind) the horse is quite capable of warming up or cooling down all on it's own. Nature designed the horse to be able to increase and decrease warmth of it's coat. The horse can control almost every hair on it's body. Ever notice how horses that have been lucky enough to not be covered by a rug on a cold day will often look like shaggy or wooly? Or how some horses will look more shaggy, as if they have more hair, then other horses in the same field on the same cold day? It's not because one has more hair or is just a woolier horse. It's because one felt colder than the other. Horses push out their hair to catch and hold air, which is warmed by their body heat, which forms an insulated layer to hold heat in. The more they push out their hair they thicker that layer of insulation. To cool it down a bit they just lower their hair a bit. Just like we could put on a heavy shirt over a light one, and then a sweater, a jacket, a heavy coat to keep warmer or take some of it off if we got too warm to avoid sweating in the cold. If the horse can't get warm enough using just this method they will use physical activity to generate more body heat too. And if they are on a proper equine diet (i.e. a lot of long fiber.... hay, grass, beet pulp, etc....) they will generate extra heat just from digesting all that fiber (remember that contrary to what humans think is best to feed them, nature designed equines to live on large amounts of low quality food (for a variety of reasons....nature doesn't do things for nothing). So horses are able to manage for themselves if provided what nature would provide and left to deal with it, unless we stick our noses in and mess with the horse is naturally able to do for itself.
Second. We a horse is rugged it is incapable of regulating it's own comfort level. It cannot move it's body hair up or down. It is stuck with whatever some caring human has determined it needs (even if it's too much or too little). Let me put it in perspective. Your hand is in a fist and it glue all the fingers in that position then cover your hands with large mittens (now you are like a horse and unable to remove whatever I put on you). Also you tongue and teeth have been removed and your larynx has been damaged so that you can't articulate anything even remotely resembling a word or an intelligible thought. i.e. you've become like a two legged horse LOL.
Now "I" think it's cold and I was brought up with the understanding that if it's cold "you" (and those like you) need to be covered when it's cold. Of course, not all of those like you have the same physical handicap I've placed on you....(i.e. they can put on or take off layers of garments to keep warm or cool down). But you are my responsibility and I'm care about your wellbeing so I'm going to make sure to do what all the "experts" have told me to do for you when the weather gets cold. Now I could always just free up your hands and allow you to dress yourself as needed since you will know how cold you do or don't feel (far better than I will know how you feel), but that's not how it's going to be, because I know what I'm suppose to do and I'm going to do it. "I" will determine what you need, "I" will give you what you need, and "YOU" will just have to deal with it regardless. If you get too cold in what I've covered you with you can run around some (hopefully I've given you room for that), but it you're too warm...tough...try to avoid sweating, it will cool you down, but then you'll really be cold. Does anyone really think that this is a good situation for a horse? Unable to do for themselves what nature has designed them to do?
Finally, there is a price that's paid for our interference. If you rug a horse enough they can (and do) lose the ability to regulate themselves (i.e. they're not able to push their hair out enough to create the thermal layers that keep them warm, because the blanket stopped them whenever they might have tried so it shuts down). Now usually this temporary and not permanent. In a matter of weeks or months it should return (I'm unaware of it ever being permanent, but I suppose it might be possible to destroy their ability forever....imagine spending your life with me dressing you for cold weather based on what "I" thought and you have to just live with too much or too little, but you cannot do anything about how I dress you for the weather).. Now that would really suck wouldn't it.
So keep this in mind when you're out there doing "what's best" for your horses. Is it REALLY what's best for them or just what you and a lot of other misguided people have be lead to believe is best? Like feeding your horse grain (oats in particular). Humans have perfected the system of killing our non food animals with kindness. Makes us feel better as we shorten their lives, because we are certain that "we" know best. After all (as I said before) we humans have been dealing with horses (other than as something we hunted for food) for over 5,000 years while Mother Natures development of their survivability has only lasted for 10 million years. Clearly we our ideas are better would have worked better than Mother Natures (did you see on those cave paintings how ancient, prehistoric man but rugs on those horses when the weather got cold. Don't you see all the reindeer herders blanketing those poor freezing reindeer to keep them from all freezing to death. Rabbits, deer, etc.. in the colder regions (MN, WI, MT, VT, ME, ND, IA, WY, SD, NY, etc...and of course all of Canada) must all be migrating south starting on Labor Day since they don't have some kind human to capture them, rug them, and then release them. Mother Nature knew and knows what it's doing. We should probably stop and take a lesson on that while learning a bit more about or our horses then what some erroneous European traditions tell us. Ever hear about how the Mongols (one of histories greatest horse societies) conquered Russia during Winter (the only time it's ever been done....and the Mongols did it in Winter on purpose, because the marshes, swamps, lakes were frozen so they could just be ridden over). Did you read about the logistical nightmare it was to keep all those Mongolian horses rugged during a Russian Winter LMAO
They're always going to be bigger and stronger so you better always be smarter. (One of my grandfather's many pearls of wisdom)