In my area of coastal Texas, we bask in mild temperatures in the 60's well into December. However, by then, many other areas have already seen snow and will see a lot more over the course of the winter.
So how do horses keep the elements at bay? In actual fact, the horse's body is well equipped to deal with temperatures way below the human comfort zone. Often horse owners will arrive at the barn, bundled up in layers of clothing, to find their horses frolicking in the snow quite oblivious of the sub-zero temperatures.
On the Outside
The most obvious concession to the season is the horse's winter coat, which can start growing in around late August in some northern areas. As the days shorten, the "hair factor" kicks in. In addition, sudden cold temperatures such as a cold front will prompt the horse's body to start producing his winter coat. Here in Texas, I have noticed my horse, Annapolis, starts to get a little hairy in about mid-October. In spite of the fact that it is often in the 70's, the shorter days and a couple of cold fronts coming through are enough to convince Annapolis' body that it's time to don the winter woollies.
The horse's winter coat differs from the summer coat in that the hairs are longer and coarser. The horse is able to fluff up his coat, the individual hairs standing out, rather than laying flat against the skin, trapping air close to his body and thereby insulating him from the cold. The additional grease which accumulates in this thicker coat, especially in pastured horses, gives additional insulation.
On the Inside
Nature has designed the horse to withstand the cold from the inside too. Horses are grazing animals that fare best if there is a small amount of food constantly being processed by the digestive system (hence the old adage "feed little and often") As the gut digests the fibre in the diet, heat is generated, contributing to the horses body warmth, even in freezing temperatures. One of the most natural ways to help a horse keep warm in the winter is to allow him ad lib access to good quality hay.
Today's domesticated horses are more prone to impaction colics in the winter. This is due to the fact that they are often confined to stables in inclement weather and receive less turnout and exercise, while still receiving a full grain ration. The fact that horses often drink less in winter, when the water is at or near freezing, increases the chances of a colic.