Two swans with their necks entwined. At the feet of the effigy of Margaret de Bohun, Countess of Devon, in the fourteenth century tomb she shares with her husband, Hugh de Courtney, Earl of Devon.
When swans are paired up (supposedly monogamously and for life) they do engage in their own version of necking, but I have my doubts that the contortions go quite so far as portrayed here.
A swan was the Bohun family heraldic beast, at least according to Wikipedia. The tomb is in the west transept of Exeter cathedral.