“To reduce weight, protect the mail, and speed up relays, Mr. Russell had special Pony Express saddles and mochilas made. The saddle was only a light wooden frame, with horn, cantle, stirrups, and bellyband. The mochila (pronounced ‘mo-chee’-la’), or mantle was an easily removable leather cover that fitted over the saddle, with openings to let the horn and cantle stick through. At each corner of the mochila was a cantina, or pouch, for carrying mail. These were fitted with locks, and the keys would be kept only at Salt Lake City, San Francisco, and St. Joseph.
Each mochila would be carried the full length of the line, being moved from pony to pony as relays were made. Since the rider would be sitting on it, it could not be lost or stolen while he was mounted. If he were to be thrown or killed during his run, the mochila would remain on the saddle and, no doubt, be carried on to the next relay station by the riderless pony.”