Rough Riders, a name coined by William F. Cody ("Buffalo Bill"), for use in his "Wild West" show, which included a "Congress of the rough riders of the world." The first rough riders were the men who carried messages over the West in the early frontier times before the pony express was organized in 1859. In the army the original rough riders were the 1st Regiment of United States Volunteer Cavalry, organized for the war with Spain by Surgeon Leonard Wood, who was commissioned colonel, with Theodore Roosevelt as lieutenant-colonel. The name was applied also to the 2nd United States Volunteer Cavalry. The name was given to these regiments on account of their being composed largely of Western ranchmen.
Entry from Everybody's Cyclopedia,