The Rough Riders were a volunteer cavalry regiment that fought in the Spanish-American War. "Rough Riders" was not an official name for the regiment. The regiment was officially the 1st United States Volunteer Cavalry. It was given the name "Rough Riders" by reporters who were following the regiment. The men of the regiment were recruited mostly from states of the Southwest United States.
The regiment became famous largely because of the fact that Theodore Roosevelt was its second-in-command (and later took command when the actual commander was promoted). Roosevelt was already a fairly high government official and was well-known enough that his regiment attracted a great deal of attention.