A shepherd herds and manages sheep, usually to raise them for an annual sale at a meat market. When used as a verb, the word shepherd means to guide something (like sheep); move them in the direction of where you want them next. For sheep, that often means from their pen to a field of grass where they graze. Thus, the shepherd shepherds the sheep to the field from their pen.
A pie consists of pastry formed into a bowl that will contain a staple food product that the baker chooses (meat, vegetables, or fruit). With that much done, the product could be called something other than a pie. When the baker adds a pastry cover (top or lid) then bakes his or her creation, he or she has baked pie. The name shepherd's pie implies that mutton (the meat of sheep) got baked into the pie. While true, if the baker chooses to use mutton, the name shepherd's pie implements to any edible food shepherded into the pastry bowl.
You know the name for prepared food products that were not all consumed: leftovers. Shepherd's pie, a freshly-baked presentation of kitchen leftovers, provides a baker with an array of opportunity to spruce up an otherwise B list meal while essentially clearing the refrigerator of older, yet useable food before it spoils. I have seen the name "cowboy's pie." A pie will eat (taste) the same if the name changes; the pie itself remains the same. Cowboys herd cattle, as shepherds herd sheep. Some fisherman herd fish into a trap, so you might have fisherman's pie if you wish, and you probably should call it that if you baked leftover fish into your pie.
If from Argentina, you may refer to your pie as gaucho pie, since a gaucho (the Argentinean word for a cowboy) herds cows while on horseback. Speaking of cows, western Americans prefer the name cattle, and they detest the term boy when referring to themselves and their tradition of herding cattle. They like cattlemen better. Cattlemen's pie works then, does not it? It also sounds better than cowboy's pie. Probably, we should let go there, because cattlemen do not typically herd cows, which live in barns, produce dairy products, and make little cows (calves).
Cattlemen herd steers to the meat market. Steers, young castrated bulls, must be used up before they become oxen (massive feed-consuming beasts once used to haul heavy burdens in carts). We do not need oxen anymore because we have trucks to haul heavy burdens. If you like to eat steak and hamburger, then you understand the use of steers. But, you probably do not want to think about a pie made with leftover castrated bulls. # TAG1writer