El Petizo is a shadow creature that is known for attacking lone walkers in a northern area of Argentina. Several people have even been medically treated for wounds received during their El Petizo encounters.
The attacks appear to persist until a rescuer comes to the victims aid, at which time the mysterious creature simply disappears.
In one of these instances two people saw the creature - the victim and the rescuer. Rense.com has a good account of it:
"The most recent victim of an attack by "El Petizo" was a boy heading to a mountain area on a bicycle to visit friends and do some hunting, bringing with him a shotgun and a knife. According to his story, after asking for reservations to be made in his name, he was toppled from his bicycle and found that a black shadow was facing him. The young man managed to unholster his shotgun and fire two shots, but when he saw that this had no effect, promptly drew his knife and stabbed it without causing any apparent harm. Frightened, the youth tried to escape, only to find that "El Petizo" knocked him down again without saying a word, dragging him by the hair to the side of the road. The young man stated that at that point he began screaming at the top of his voice and that this is what saved him: his cries were heard by a local man who rushed to provide assistance, only to find a black shadow dragging the boy by the hair. The fearful shape disappeared without a trace when it became aware of the other man's presence."