Here's an excerpt from a website on Western American history:

Mule Skinners and Freight Wagons

The Mule skinner was a professional individual sometimes called a teamster whose sole purpose was to keep his wagon pulled by mules, under control and moving. The mule skinner actually rode one of the mules and guided the entire team with a single rein which was called a jerk line. An experienced mule skinner knew the personality of every one of his mules and could make them into a magical running machine whereas an inexperienced teamster found them to be obstinate and stubborn

Speed was of essential importance out in the west and the mules could pull wagons at 2 miles per hour. A team of oxen usually pulled at about 2 1/2 miles per hour. General stores would specify mule teams to carry their freight of food and other perishable items. The draw back to mules were that their grain had to be hauled with them, the Indians would steal mules to ride them and mule meat tasted terrible according to the teamsters

During the 1800's the mule was in constant demand for civilian and military freighting. Not only were the mules better foragers, they kept better footing in treacherous terrain then the horses. A pair of mules could cost anywhere from two to four hundred dollars during this time period. Of all the mules, the ones from Missouri were the most prized and the reputation continues even today

Mules were also used to pull the stagecoaches on the western end of the stage run, but the men were still referred to as stagecoach drivers

The most famous mule teams were the Death Valley twenty mule teams that hauled borax across the desert to the railroad. Each team actually had 18 mules and 2 horses as the wheelers