When it is very cold, horses can get very stiff, just like humans. In order to avoid this, try to work the horses as much as possible.
When working your horse, you can try to make sure that it does not get injured by doing two main things. One is major stiffness and arthritis, the prevention against which is provided by a supplement called Corta Flex, which can be found at many equestrian supply stores.
The other thing you can do to minimize injuries in the winter would be to warm up your horse thoroughly before riding sessions.
When riding the horse in cold envirnment, warm up the bit before putting it into his mouth. To warm the bit up you can use a heater and place it near for a few moments. Or, you can breathe warm air onto it and rub it through your hands.
After getting off of your horse, if you think that you will not be able to remove all the tack immediately or will be walking with him for a while in the cold, then place a cooler on him. A cooler is a lightweight blanket that will keep him warm and prevent him from cooling off too quickly after having been worked.
Without the cooler on, the horse could easily get uncomfortable because its sweat will dry too quickly.
After a workout with your horse in the winter, it will probably be much too cold to rinse him off, so make sure you cool him down with a thorough curry.
One of the last steps to take care of a horse in the cold of winter is making sure that it has a heavy and warm blanket on, whether outside or inside. Make sure that the blanket is fitted correctly because an ill-fitting blanket might risk hurting your horse - he could twist a muscle inside it or get his circulation cut off becuase of tightness. Wearing this blanket will keep your horse warm and comfortable.