A donkey is an animal that is often used for pulling or carrying heavy items. Male donkeys are often bred with female horses to create a mule.
On March 3, 1850, Marty McFly, Jules Brown, and Verne Brown disguised the DeLorean time machine as a covered wagon.
When Daniel Clayton and Martha O'Brien's wagon train arrived, they loaned let them join the wagon train, and lent them two mules.
Wild Bill Tannen kidnapped Verne and Martha O'Brien. He blinded himself with a camera, and was kicked by a donkey, giving Martha time to untie the ropes that held herself and Verne.