A fan named the Denver Broncos in a contest held when the professional team initially formed in 1960. Denver's Midwest Baseball League team was also known as the Broncos in the 1920s.
Ward M. Vining submitted the winning entry with his 25-word essay on why Denver's American Football League team should be named the Broncos. He was among 162 fans to submit entries. The name spoke to the city's "wild West" heritage, according to an article at MentalFloss.com.
The official Denver tourism website indicates that the city became an epicenter of America's old West, in large part owing to the discovery of gold in the mid-1800s. As a result, horses, including a fair number of untrained broncos, were a fixture in the region.
FootballHappenings.com reports that the Broncos, which later became part of the National Football League, are the only NFL American Football Conference West team to never have changed its city of residence or its name. It is represented by two mascots, one of which is Thunder, a purebred Arabian horse. The other is Miles, a human being dressed in costume to resemble a horse. The name "Miles" is reflective of Denver's nickname, "the Mile High City," a reference to the elevation at which the city sits.