A 'corrido' is a ballad, an offspring of the Spanish 'romance.'
The octosyllabic verses of a romance were changed to quatrains, "more pleasing to Mexican estheic sensitivity" - Frances Toor, "A Treasury of Mexican Folkways."

Original verses? I don't know either, I haven't the proper references, but I may find some in Cantos populares Españoles, which I haven't looked at yet. So far, those I have all concern the revolution of c. 1910-1920 and concern the cockroach.
No idea of what the original Spanish song was about, except that is concerned the war with France.