Veterinary treatment may be needed. Therapy aims at preventing or controlling shock, neutralizing the venom with antivenin injections and preventing tissue necrosis and secondary infection. Your veterinarian may provide an antivenin as well as antibiotics to prevent infection. He'll also administer anti-inflammatory drugs to reduce tissue swelling. A tetanus booster is in order.
Avoidance is the best treatment.
When walking in deep grass, make noise. Snakes generally are not a threat and want to be left alone, so if you see one, find a way around it. If that isn't possible, wait it out, or use a long stick and gently encourage it to move. If the wild critter is rabid and bites, there's a good chance you'll lose your horse - unless he's had a vaccination. People will vaccinate for tetanus, influenza and encephalitis, then neglect rabies.