Although there have been other riders to carry the mail by horseback (Benjamin Franklin established post riders that carried mail in the East), it was the mochila that made the Pony Express unique. Mail pouches were never in use on the Overland Pony Express because of their size and shape, they would be difficult to attach to the saddle, and would cause undue delay in changing mounts. To get around this difficulty, a mochila, or covering of leather, was thrown over the saddle. The saddle horn and cantle projected through holes which were cut in the mochila. Attached to the broad leather skirt of the mochila were four cantinas, or boxes of hard leather. When the rider was in the saddle, his legs came between these boxes. The mail was put in these cantinas, which were locked with small padlocks. Station keepers at either end of the line, or at designated way stations, had keys to these boxes.
Through the use of the mochila, it was not necessary to change saddles at all, properly speaking. When a rider arrived at a relay station, a fresh horse, saddled and bridled, was waiting for him. The rider changed the mochila from one saddle to another and was away, inside the two-minute limit allowed for changing horses. If a horse fell and was killed or injured, the rider could strip the mochila from the saddle and walk to the next station with the mail.
The saddles were made by the famous saddlery firm, headed by Israel Landis. They were light even to the stirrups. The saddle horn was short and broad and the entire saddle weighed about one-third of the ordinary frontier used saddle.
Since the riders only had two minutes to exchange horses, this design allowed for easy and quick removal and placement on a fresh horse at Remount Stations. The rider's weight kept the mochila in place during the rugged rides. The mochila, saddle and bridle together weighed 13 pounds.
The National Pony Express Association uses a mochila designed to carry a greater number of letters and to fit over a western saddle, yet can easily be transferred to a fresh horse at rider and mochila exchange points during the annual Re-Run.
See also Roy Bloss's essay "Thomas Bowdler's Elegy For The Pony Express."