Martín Fierro is an epic poem (and its hero) written in 1872 by the Argentinian poet José Hernández. In it, a gaucho plays the guitar in a tavern, singing the story of how his life was destroyed when he was conscripted by the army.
Fierro is drafted, beaten up; his possessions and horse are stolen; he's taken off to the frontier to fight Indians, but really ends up working on his officer's ranch for no pay; his children starve as beggars and his wife disappears. When Fierro escapes the army, he drinks a lot and gets into fights, killing several people and thus becoming an outlaw.
Fierro repeatedly laments that he was a hard-working and peaceful cowboy before society's injustice turned him into a bitter outlaw. He eventually goes off to live with the Indians. The free and independent life of the gaucho roaming the pampas is romanticized throughout the poem.
The sequel is The Return of Martín Fierro, and blends in pretty seamlessly with the first part of the poem.
Basically, Martín Fierro is a rap star.