A brujo is a male witch, descending from the Spanish word brujería (witchcraft).
As an amalgamation of various beliefs and practices, the definition of what constitutes a bruja was not fixed, but amongst Hispanic cultures the term invariably held negative connotations. The practice of brujería was feared as a manifestation of evil, and to refer to somebody as a bruja or brujo was essentially to label them an ‘evil doer’.
Don Arnulfo's name comes from the Scandanavian Ernouf, or Arnulf, meaning wolf eagle. This is significant not only because he helps Billy catch the wolf but also because the brujo were considered accomplished shapeshifters, possessing the supernatural ability to transform themselves into various animals.