Like the lightweight sweatshirt in your gym bag, a cooler is a useful part of a working horse's winter wardrobe. Made from warm but breathable materials, coolers control the rate at which a hot horse cools down, reducing the risk of chills. Most coolers do this in three ways:
by wicking moisture away from the horse's skin and onto the fabric's outer surface, where it evaporates.
by blocking the wind, which will literally blow away the heat produced by the horse's body.
by trapping and retaining warmth between the fibers of the fabric to act as insulation.
To get the greatest benefit from a cooler, toss it over your hot, sweaty horse and walk him until his pulse and respiration rates return to normal. Then, keep him in a draft-free area, with the cooler on, until his skin feels warm---not hot---to the touch. But be watchful while he is wearing a cooler: Some do not secure as well as regular blankets, which makes it easier for a horse to become tangled.
When your horse's coat is mostly dry and his skin feels nearly normal in temperature, you can put on his regular blanket and call it a day.