Prior to the 1880's, there's no mention of this trait in goats anywhere. In the early 1880's, a guy named "John Tinsley" - a vagrant farmer, moved into Marshall County, Tennesse, bringing 4 goats (3 does and a buck) with him that had this condition.
He later sold those goats to Dr. H. H. Mayberry. These goats were bred and sold off to cattle and sheep ranchers who found the trait valuable - the goats served as decoy animals in case of any preditors. Thus, this condition does not offer them some evolutionary advantage to avoid predation.
The only "evolutionary" advantage is that the condition has caused them to be selected by humans, as has been pointed out in this thread. (Ditto for poodles. Mother nature *certainly* didn't want those horrible yapping things to exist.) But, here are some of the reasons that humans selectively breed them:
1. The condition makes them a very easy breed of goat to raise; they don't have the jumping and climbing abilities that many other breeds of goats have. (And, having a couple Nubian goats, I completely agree with this. I'm getting rid of the Nubian buck for this very reason - he's gone over the fence in with the girls and cross-bred one of my fainting goats. He's a p.i.t.a.(but friendly). Our Nubian female is also friendly and also a p.i.t.a., but one that we're going to put up with. The other night, she leaped over 2 or 3 among the 30 other goats who had surrounded my wife (who was carrying a bucket of grain), taking out my wife who disappeared beneath a sea of 30 fainting goats, with her arm weakly and futiley attempting to hold the bucket of grain out of the reach of the goats. I couldn't help her. I was laughing too hard. Within 3 seconds, there was no sign of my wife; just a mass of goats attempting to get to the bucket. Yet another America's funniest home video $5000 moment not captured. But, I digress... back to the reasons:
2. They have a very high ratio of meat to bone, making them an excellent choice as a meat goat animal, or to cross with other breeds (boar) to improve their meat characteristics as well.
3. Their condition leads to more tender meat (or so I'm told; I haven't had fainting goat meat.)
4. Better personality and more docile nature than many of the other breeds of goats. Again, my personal experience confirms this. Even our fainting goat/nubian crosses born here have very obvious behavioral differences to the fainting goats born along side them. These behavioral/personality traits make them a superior pet goat. We leave the
horns on our fainting goats (Ahmand is the exception; he's naturally polled. Skye is just a youngster, so his horns aren't too long yet.) Other breeds of goats routinely have their horns removed when they're little. We raise our fainting goats for: preservation of the breed, education, pets, and of course, breeding stock for ourselves and for others. Right now, we have somewhere around 50, including the two nubians and two nubian crosses; and we're expecting about 6 more to be born soon.
The condition doesn't only appear in goats - similar conditions exist in other animals. However, the condition in fainting goats is most similar to myotonia congenita in humans (Thomsen's disease) and dystrophia myotonica.
You can find a lot more information
here Also, if you google for Dr Phillip Sponenberg, he's been doing quite a bit of research at Virginia Tech on the fainting goats, and is also a breeder. I don't know what kind of research he might have posted online, or what type of research.