lobisón : a werewolf, or the Argentine version thereof (metamorphoses into a cross between a pig and a wild dog, as there are no wolves in Argentina); the transformation occurs every Friday at midnight, independently of the phase of the moon. Favorite diet is "excrementos" of every type, spiced up occasionally with the flesh of an unbaptized infant.
As noted previously, being a lobisón is the fate of any seventh consecutive male child*. Physically, the untransformed lobisón is thin, subject to digestive ailments, with a propensity for staying in bed on Saturdays. Not terribly surprising, given his nocturnal activities on Fridays.
*For extra credit: Juan and Begoña are a Patagonian couple with five strapping sons (and no daughters). Given that they are devout practising Catholics and have two further children, what is the probability that their seventh child is a lobisón?