According to Wikipedia, a teamster was originally (and not surprisingly) "a person who drove a team of draft animals, usually a wagon drawn by oxen, horses, or mules."
Wikipedia goes on to say that "This term was common by the time of the Mexican–American War (1848) and the Indian Wars throughout the 19th and early 20th centuries on the American frontier ... Another name for the occupation was bullwhacker, related to driving oxen. A teamster might also drive pack animals, such as a muletrain, in which case he was also known as a muleteer or muleskinner. Today this person may be called an outfitter or packer. In Australian English, a teamster was also known as a bullocker or bullocky."
Today, the USA's largest trade union – representing transport workers – is famously known as the International Brotherhood of Teamsters (IBT). It was founded in 1903.
© Haydn Thompson 2017