Your dogs are much more at risk of being bitten by a rattlesnake on the trail than is your horse. So, I suggest sending your dog through a snake avoidance clinic. We send all our search & rescue dogs through these clinics so that we can avoid the issue of a dog bitten by a snake.
As for your horse, unless it is bitten in the face (which should not happen on the trail), it should recover with no issues. And even if bitten on the nose, the issue is swelling that might block breathing (which can be easily handled by putting an 18" length of regular garden hose up their nostril to permit breathing for the few hours of the swelling).
Here is the website for the major snake avoidance clinic in California.