Do not try to trap the attacker. This is a dangerous waste of time. Your veterinarian does not need to know the specific kind of snake that bit your horse to begin treatment. If the snake is trapped in a structure, call a professional to remove it. And keep dogs and cats away from the snake--because of their small size a bite could be fatal for them.
Do not try to remove the venom. Despite what you may have seen in old Western movies, trying to "suck" the poison from a snakebite is a really silly idea. It does no good for the horse, and there's even a small chance you'll do yourself harm--venom degrades in the stomach but if you have a cut on your mouth, it could enter your body.
Do not apply a tourniquet. Cutting off circulation to a limb may precipitate other problems without effectively preventing the toxins from spreading through the body.