"Some theories consider that the balance of good and evil, created in church design to remind worshippers of the narrow path they tread, was present in everything. This meant that for every good and benign creature such as a saint or an animal to signify purity, there had to be an opposite to bring out the fear of evil. In York Minster, for example, the carvings in the Chapter house, which are particularly disgusting and obscene and which were supposedly created as caricatures of the then Dean and Chapter, were put there above the seats to create an opposite to each occupant, who we might like to assume was not in fact the foul person their carvings made them out to be."

wikipedia