los
/los/ · A1 · pron
plural masculine or neuter pronoun
los = el + s: Latin illos lost its head, kept the -los ending
From Latin illos (those, them), the same root that gave el and lo. As an article it is the plural of el (los libros); as a pronoun it means them (los veo = I see them). The final -s is the ordinary Spanish plural marker.