Horses are very prone to wounding themselves and sometimes the wounds look horrific and bleed profusely.  Wounds on the body actually heal surprisingly well but large leg wounds may prove more difficult to resolve. 

Initially the bleeding must be controlled, although some bleeding may be useful to flush and clean the wound.  It is important to contact your vet if the bleeding is profuse or if the wound is deep.  In addition, a very lame horse, even with a small wound, should be examined especially if the wound is near a joint.  

Try to control the bleeding before the vet arrives by applying firm pressure to the area with a clean pad, or gamgee, or in an emergency whatever is to hand e.g. handkerchief, t-shirt etc.  Apply the pressure for more than 5 minutes and try to keep the horse as calm as possible.