Perhaps it's time for the clever people at JPL and other places to design some kind of electric horse. They've designed successful rovers for the Moon & Mars. And we have had horseless carriages for as long as I can remember. Lots of markedly better battery and motor technology has popped up in the last few years, too.
After all, the requirement is only to reliably carry a few hundred pounds a few dozen miles.
Then there is the Llama - which is far better adapted to altitude and easier on the environment.
Why is the horse the only solution?
The horse is not the only solution, as you mention with the llama, and there are those who use goats, as well.
However, the horse is indiginous to the region, and the others are not. Occasionally, a horse gets loose in the backcountry, and they sometimes take a month to get back, and survive ok. I don't think a llama or goat will fare so well. Neither do dogs, by the way.
BUT, the idea that you mention, was first actualized in 1958!
THE TOTE GOTE!
http://en.wikipedia.org/wiki/Tote_GoteThe use of this was described in depth in Chuck Yeager's book "Press On", where he used them in the Whitney region backcountry!
