What was the most common cause of early death for cowboys in the old West?

Hint, not Indians. Or other cowboys. Although gunfights did happen, they were relatively rare.

Nope.

The most common cause of death was falling from a horse. Riding helmets had not been invented (and sadly, many modern cowboys refuse to wear them out of a sense of tradition), so head injuries were common. So was being dragged by the stirrup, although good boots could prevent that.

Dying by falling from a horse would also have been a common fate of mounted warriors - the most common training accident would have been a simple fall. A knight's helmet was designed to protect him from blows to the head, so might have helped reduce the number of brain injuries - or not. And, of course, horse trainers would have faced this as their greatest risk (and to an extent still do).