Your horse comes in from pasture and his nose is swollen like a balloon.
While not common, snakebites do happen in some parts of the country, and are particularly prevalent in summer when the snakes are sunning themselves in grass and along rocky trails.
If you're present when your horse is bitten, immediately dismount (if you are riding and after the snake has gone, of course) and call your vet to see if he can meet you. It's important to keep the horse calm and as still as possible. If you're on the trail, walk the horse very slowly and with his head down-venom moves through the bloodstream so the idea is to keep the horse quiet and the venom from spreading too quickly.
If you ride in areas with a lot of poisonous snakes, consider carrying a length of hose and lubricant to insert in his nostril should he get bitten on the nose and you're far from help. Don't cut the wound and try to suck out the venom. This is an old wives tale and could make things worse.
If possible, identify the snake's markings. This will help the veterinarian know what kind of treatment to undertake. He will give the horse antibiotics, anti-inflammatories, and possibly also an antivenin drug. The good news is that most horses recover just fine from snakebites.