Here's what I believe is the meaning of the "Six White Horses:"

The white horse/horses are symbolic of traveling from life to death in the African-American culture. We sing an old spiritual with "two white horses in a line, waiting to carry me to d'other side."

Since the term, "two white horses" or "six white horse" is a common line in blues/spirituals it was appropriated in the "She'll be Coming Around the Mountain" song.

In the context of the song I don't think "six white horses" means anything but was a line combined in typical folk metamorphosis from the African-American expression. It gives her a kind of supernatural power (over death?) to "ride six white horses."

-Richie