I had a mare bit right on the nose a few years ago..... when she was due to foal in one week. If I had not found her within about 3 minutes of it happening, and got a tube inserted in the nostril on the good side, she would have suffocated by the time she got to a vet.
By the way folks, I keep aquarium tubing on hand as a garden hose is much too big for a mini and not very flexible. I keep the 1/2 stuff on hand and it worked great for the mare. Would probably be too big for a foal though. I cut a piece from my aquarium vaccuum cleaner which only cost a few bucks- well worth buying another one if need be!!! It was taped to the halter and yes, her head swelled HUGELY.
There is not a whole lot a vet can do, and they are going to swell regardless. The main thing is to keep them breathing. However, it depends on where the snake bites them, and how badly. There are different kinds of rattlers but venom is made up of things that can cause necrosis of body tissue, neurological damage and organ failure.
We have had some, and dogs, survive, but my neighbor did lose a young mare to renal failure. She just gradually deteriorated over the days..... and didnt make it. It can affect all organs though, so if your horse continues to not want to eat, or gets worse, get her seen by someone a.s.a.p.!!!!!
My mare, though her face was unrecognizable, she could not close her mouth, had to breath through the tube for two or three days before the swelling started to go down and the tube fell out (yeah!) never lost her appetite, so I would say to beware of that as a possible symptom of another problem.
I sure hope your little horse recovers ok.... this is always a serious issue and pretty scary until they are better and healed up.
P.S. The mare foaled one week after she was bitten, and by that time looked like herself again and had no problems foaling! The foal was named Snakecharmer and is owned by a farm in NY!!