If the whole ankle seems to sag towards the ground, with the fetlock of one leg lower than the other, there has been injury to the suspensory apparatus. Both of these injuries require either a gelocast or snug standing bandage immediately to prohibit swelling. Apply the gelocast as a tight bandage, starting below the knee, down to the pastern, and up again as many times as necessary to use the whole bandage. These tissues heal better if there is a minimum of swelling, unlike some other injuries. Your veterinarian will want to examine your horse and prescribe a course of treatment.
Horses can get common colds just as people do.
Keep him warm and dry, adding a blanket if necessary. Allow him plenty of fresh air but avoid direct drafts. If your horse is coughing, give him some equine cough syrup. You can also use Vicks Vapo rub in his nostrils or the equine equivalents called Vapol or Trail-eze. Your horse may develop a splint.