what I haven't seen here, is any referrence to the classic country song Six White Horses...Waylon Jennings. In the song he's the blind father of a young soldier in, I think, the Civil War.

"Come here and look through the window, Marie. Open up the shutters and tell me what you see. Was that his knock I heard at the door? Or was it six white horses coming up the road?"

I'd have to say that in that particular song, it either refers to a military party to notify him of his son's death, or actually bringing him the body.

So, I agree that, in some cases, the significance of six white horses has to do with death, but I don't think in She'll be comin round the mountain, that's the case.