Pack saddle

Military style pack saddle with a swag (bedding) on top
The Sawbuck pack saddle, traditionally used in the western United States
American 20th Century, Unrigged Pack Saddle, c. 1932, National Gallery of Art

A pack saddle is any device designed to be secured on the back of a horse, mule, or other working animal so it can carry heavy loads such as luggage, firewood, small cannons, or other things too heavy to be carried by humans.