Yes, it is his only job, not a veterinary, not a breed assistant. And just for horses. In sort, that guy: http://www.hoy.es/noticias/200809/23/Media/germoplasma.jpg Etimology: from "mano" (hand) and "porra" (cock). In the past, that was a honorable profession, because a good mamporrero provided good breeds, selecting horses and ensuring a correct intercourse without hurts for both horse and mare. Nowadays this job is made by specialized veterinaries, and in a figurated and despective way, also means "who do the dirty work to someone".