Shepherds' Pies comprise a layer of pre-cooked minced lamb and vegetables, topped with mashed potato. So not actually a pie; more like 'Shepherds reheated mince-and-mash'. Also the word 'lamb' in the UK doesn't refer to meat from a sheep; it is a courtesy title given to meat from horses, donkeys, stray dogs, or seagulls.
Shepherds' Pies were originally proper pies made from pastry. In 1066 King Henry the Eighth (The Black Prince) was dissoluting monasteries left right and centre when a lowly shepherd approached him attempting to highlight the risks inherent in destroying the cultural record. King Henry was so incensed at being spoken to by a prole, he banned all shepherds from eating pastry for evermore, and the Shepherd's Pie was borne.