Actually, most horses don’t mind cold weather if they are healthy, dry, well-fed, and have access to shelter from the wind. These are crucial “ifs,” however.
Health status is important. Very old, very young, and very thin horses may need additional care (stalling, blanketing (rugging), extra meals) in extremely cold weather, as may those that have recently been brought to the colder place from a location with significantly warmer weather. Horses recovering from neglect and starvation won’t be able to resist the cold as well, and horses whose teeth are in poor condition may not tolerate low temperatures because they can’t eat efficiently.
According to Bryan Waldridge, D.V.M., staff veterinarian for Kentucky Equine Research, keeping horses dry is a huge factor in keeping them warm. Waldridge explained, “When a horse’s coat gets wet, it loses its protective loft. Body heat is then easily sucked away by the air, especially on a windy day.”
Much of the horse’s body heat is produced by the fermentation of fiber in the hindgut. Grain provides extra calories, but to keep horses warm in winter, they need to have a steady supply of hay to keep the internal fires stoked. Be sure that the horses always have salt and fresh, clean water that is not too cold (45-55 F or 7-15 C is more palatable than ice-cold water).
Horses should be given access to shelter, both for staying dry and for getting out of the wind. This can be as basic as the lee side of a barn or as fancy as a large covered structure with deep bedding and hay racks. Some horses will prefer to be inside and away from rain or snow, while others will probably stay out in the weather by choice. As long as shelter is available, you don’t need to force them to take advantage of it.