For another twist.An expedition for any reason can use all the help it can get hauling heavy loads.They were known to ship livestock.Why not take a few horses along to carry the goodies?
I had never heard that suggested, but a year or so ago in Ancient American there was an article on a Wisconsin Indian Mound that was excavated some 120 years ago, and had a horse's head in it at least, if not the whole animal, from a clearly pre-Columbian date.
It is becoming thinkable in other words, to argue that some horses were here in America before the Spanish lost a few in the 1500's...I always did think that very large herds indeed of wild horses were in the West awfully soon, to have arisen from just the few the Spanish could have lost...