Thehorse.com - Full Article
by: University of Maryland College of Agriculture and Natural Resources
November 18 2008, Article # 13124
The short answer might very well be "to blanket." However, for many horses blanketed during cooler months, that extra layer provides more comfort to their caregivers than to the animals themselves. Horses have evolved to have an excellent built-in temperature control: a very thick winter hair coat. Horses require between 10 and 21 days to acclimatize to colder temperatures. For instance, on the first day of 35° F weather, the horse might feel cold, but over 10-21 days of similar cold weather, he will "get used to it" and be more comfortable.
If temperatures drop suddenly, you will notice behavior changes, including increased use of shelters if available, huddling together in groups with other horses, and turning their hindquarters into the prevailing wind. These are all activities that help conserve heat. The shivering response will also occur in very cold horses, which generates a pretty substantial amount of body heat, helping to warm internal organs.