The horse originally evolved as a steppe and tundra animal–a creature of colder climates. He’s highly adaptable, of course; humans have taken him wherever they’ve gone, though the North and South Poles are problematical–not enough forage for those big bodies.
But the basic animal is a subarctic creature. His optimal air temperature is around 50 degrees Fahrenheit. He can survive, with time to adapt, down below zero and well above 100 (especially if he happens to be one of the desert breeds). As long as he has some form of grass forage and plenty of water, he manages.
One reason for his adaptability to heat is the fact that he has a multitude of sweat glands all over his body–his cooling system is extensive and highly efficient. Another is that his coat adapts to the needs of the season. In summer it is sleek, short, and close to the skin; on the head (especially the nose and eyes) it may be so thin that the skin shows through. The hairs are a quarter to half an inch long on the average, and protect the skin without adding heat. In fact there’s even a mutation for the desert: the horse who is born dark or spotted and turns white by ages 4-8 on the average. The skin for the most part remains black, so that it resists sunburn while the white coat reflects heat. This coloring is called grey.
As summer winds down and autumn advances, the horse’s sleek coat begins gradually to lengthen and thicken. The summer coat will shed and the new coat will come through a bit like flocking underneath. The horse will not, be it noted, shed out completely before the new coat comes in; one coat merges into the other. The hair on the legs thickens and gets longer, and the face fills out and may gain a “beard” of guard hairs under the chin. By the time the cold has set in for the winter, the sleek creature of the summer has disappeared and been replaced by, essentially, a giant plush toy. The average coat is an inch or so long, but some breeds and individuals may grow three inches or more.
This is a great adaptation for the tundra, but for a domesticated animal who lives in a barn, it can be a nuisance. Those sweat glands don’t shut down with the winter coat; a horse who is worked heavily will sweat heavily, and the sweat stays on the long coat, and may freeze there. Horsekeepers in that situation may clip the horse all over or only on the portions that sweat when the horse is ridden, and compensate with a rug or blanket. Otherwise riders will spend hours changing out wool or fleece (or these days, microfiber) sheets or coolers as they get wet, walking the horse, and rubbing him dry.
Horses grow winter coats even in warm climates. Coat growth is controlled by light first and then temperature; when the days get shorter, the coat gets longer.
One issue that may come up in a novel is what to do if the weather is horrid. The horse’s winter coat is a marvel of engineering, but it is susceptible to a combination of wet and wind. On very cold, wet nights when the temperature hovers just above freezing, the horse’s natural defenses break down. The coat loses the loft of the hairs that traps warmth; the hard rain gets through to the skin. The horse will shiver to try to keep warm, and try to find enough forage (i.e. calories) to offset what’s lost to the cold. Wild horses are out of luck. Domesticated horses may be blanketed, though that is a heated and often acrimonious debate: whether to let the horse “toughen up” or give him a little extra help when conditions are more than his body can easily handle.
Horses can and will survive naked in very cold climates. The secret is adaptation. It takes from 10 days up to three weeks for a horse to adapt to a given type of weather. In Nebraska in January, when it’s been freezing cold for a couple of months, the horse only needs help if there’s a thaw and it gets very wet. In Arizona, where temperatures will veer wildly from the 80s to the 20s, sometimes in the course of a few hours, it’s a little harder. Horses in Arizona may see blankets more often than horses in Nebraska, simply because they don’t have time to adapt to the swings from heat to cold and back.
A cold horse or a horse flung suddenly from heat into cold is not only compromised in the immune system; being a horse, he’ll probably colic, either from the shock of the cold causing his belly to cramp, or because he won’t or can’t drink the icy water that’s all he has available. Heated buckets and water tanks are a godsend in cold climates. If technology doesn’t allow electric heaters, there’s the old-fashioned hauling of buckets from the warm house or stable, which has to be done several times a day as long as the temperature is below freezing. A warm mash is a good old-fashioned remedy for the cold horse (usually made with bran and oats or barley, maybe with sugar or chopped apples or carrots, and soaked in warm water); so is adding a little salt to the mash to encourage him to drink above and beyond the liquid in the mash.
Winter horsekeeping is labor-intensive, between keeping the horses warm and hydrated and making sure the water supply doesn’t freeze–chopping ice in the buckets and breaking ice in the water tanks. Horses will do this themselves if they can, but if the tank is frozen solid, even a hoof won’t make much of a dent.
On the keeping-them-warm front, horse blankets in the old days were monstrously heavy things made of canvas (possibly waxed for water resistance) and wool and in very cold climates, lined with fleece. Getting one of those on and off the horse is a distinctly aerobic form of exercise. A horse blanket is big–especially for a larger horse. Six feet or more long, and eight or more feet wide, not counting chest and tail flaps. For the big draft horses, you’re looking at as much as much as 90 inches in length and a comparable amount in width. That’s a lot of waxed canvas–which gets horrendously stiff in the cold.
Modern technology is much less weighty and unwieldy. The contemporary waterproof and breathable blanket (with different weights of polyfill or Thinsulate for different temperatures and coat lengths) is quite light and easy to maneuver–a significant improvement for the stablehand who has to blanket a whole barnful of horses.
Some barns, especially those that specialize in various show disciplines, require a whole range of blankets for each horse: the mesh fly sheet for warm weather, the waterproof turnout or rain sheet for those rainy days, the knit anti-sweat sheet and the wool or fleece cooler for the after-ride cooldown, the quilted stable blanket for when it gets colder, the medium turnout blanket for those autumnal or mild winter days, the heavyweight turnout blanket for when the weather gets seriously down to business…and underneath, a clipped and primped and carefully groomed animal who looks very little like the wooly native of the steppe.
All of this wool starts to shed as early as January–earlier if the horse is blanketed or kept in a warm barn–and by March the horse is in full shedding mode, with hair coming off in handfuls. The groom gets to “enjoy” lengthy sessions with a currycomb or a shedding blade and lots of elbow grease, and eventually, by April or May, the sleek summer horse is back, and that’s how he’ll be until winter comes round again.