The mystery of where Shaggy had gone ended when I found his body. I was able to identify him by the unique white markings on his left rear foot.
How did he die? He seemed too strong and healthy to have died so abruptly from disease, and I don't think that he was fatally injured in his role as a stallion defending his harem of mares.
What seems the most likely answer is that he was shot. His corpse was 600 yards away from the waterhole and near the main dirt road that travels through the desert. My guess is that some jackass with a rifle shot at him from the dirt road as he chased his herd away from the waterhole. I could be wrong, but there have been many cases of wild horses that have been senselessly shot by people in the last few years. My son and I found an old horse skull in the bottom of a wash a few hundred yards from the same road with a bullet hole in the lower jawbone. The horse may have died of starvation, being unable to eat with the wound.
Whatever killed shaggy, I was very saddened to find his remains.