The Pony Express was a way to send messages quickly from one place to another a long time ago when there weren't any phones or computers. To send the messages, people would ride on horses very fast and pass the message to another person who would ride on another horse very fast until the message got to its destination.
The Pony Express Mochila was a special bag that was used to carry these messages. It was made of strong leather and had many pockets so that the message could be kept safe and not get lost during its journey. The pockets were also labeled with the names of the different towns that the message had to go through.
The Mochila was also designed to be very light and easy for the rider to wear while riding on the horse. It was hung from a special saddle called a "McClellan saddle" that was used by the Pony Express riders.
So, to summarize, the Pony Express Mochila was a special bag that was used to carry important messages across long distances during a time when there was no other way to communicate quickly. It was made of strong leather, had many pockets, and was very light and easy to carry while riding on a horse.