If you got the possibility, make a picture of the snake. If it is a venomous snake the picture will help to determine the type of poison way faster by an expert.
If I didn't knew the type of snake, I'd call the veterinarian immediately. On humans you might want to do some blood-letting, which I do not think is recommended with horses by anyone else than professionals on that, so I doubt there is much you can do apart from the usual wound treatment.
Anything that stops the blood from spreading in the body would be helpful, but you can simply not bind off a horses leg (if you know otherwise: correct me please!), so there really isn't much one can do apart from taking all measures to have the horse relaxed and calm and provide as much detail on the snake as possible.
You might determine the snake's kind beforehand if you have the possibility and maybe you can store some antidotes or at least emergency contacts for antidotes and information on
- how fast the poison acts (how much time to act?)
- how long it takes to acquire the antidote in case of emergency
- how to transport / store the antidote
- how to apply it
All this kind of information might take to long to gather in case of real emergency, so work out some emergency plans that can be followed through step by step and fast. In a situation like this you might not be able to think as reasonable as normal, so make sure visualize yourself handling the situation calm and secure several times to be able to remain focused if it happens.