Oxen pulling wagons.
Overland travellers from the Missouri River to to California and Oregon used oxen, horses, and mules to pull their wagons. The most popular animal with emigrants was the ox. It was cheaper, stronger and easier to work than horses or mules. They were also less likely to be stolen by Native Americans on the journey and would be more useful as a farm animal when you reached your destination. Oxen were able to exist on sparse vegetation and were less likely to stray from camp. The main argument against oxen was that they could become reckless when hot and thirsty and were known to cause stampedes in a rush to reach water.
Between 1840 and 1860 more than half of the animals used to pull the wagons were oxen. Probably the major reason for this was that an ox cost $25 in the 1840s whereas mules were $75. During the early stages of this migration, mules were the second most popular animal with the emigrants. Later, horses replaced mules as the second choice for pulling wagons.
Primary Sources
(1) Heinrich Lienhard, From St. Louis to Sutter's Fort, 1846 (1900)
After a few days everything fell into a certain routine. It was easier to handle the animals. Each beast seemed to answer to its name and had learned the different commands. The danger that one or another might stray from the herd became less every day. After the camping site had been selected and reached and the wagons formed into a circle, the first chore of the evening was unyoking the oxen. Then everybody hurried to gather the necessary firewood and to fetch water, and those who had tents pitched them. Fires were lighted all the way around the circle, and soon one could hear the sizzling of steaks roasting and could smell the aroma of coffee brewing. Here or there one could see people mixing dough for biscuits and different kinds of cake. When someone was lucky enough to shoot some game - which was rare for long periods of time - the meat was usually roasted or stewed immediately. After supper was eaten and the dishes were put away, groups gathered together to gossip about all sorts of things. Some told stories; other sang; still others discussed the road ahead, the supply of wood, water, and grass, the danger of Indians, the chances for game, the purpose of the trip, and so on.