Pony Express mochila
A Pony Express mochila was a removable lightweight leather cover put over a horse saddle for carrying mail and was used by the Pony Express.
Riders of the Pony Express made quick exchanges to new fresh horses, usually within a few minutes, at each remount station on their route across the United States. The mochila was removed from the exhausted horse and swiftly placed over the saddle of the fresh waiting horse.