I know it means cow's head, but why did he have that for a name? Was it a translation from an Indian name or something? I've never encountered another Spanish name like it, so where did it come from?
If I remember the story correctly the origin of the name goes back to the Reconquista in Spain...
In 1212 Martin Alhaja, a shepherd, placed a cow skull at a key access point along a road through Despenaperros Pass. The skull marked where King Alfonso VIII could work his army around the Almohad Caliphate at the Battle of Las Navas de Tolosa. Thanks to the shepherd's assistance King Alfonso VIII won a resounding victory over the Berber-Muslim army that marked the beginning of the end of the Almohad dominance in the Iberian Peninsula. In exchange for his assistance Martin Alhaja was granted the title Cabeza de Vaca, complete with a coat of arms depicting a cow skull.