According to some food sources, you can only call it shepherd's pie if you use ground lamb. If you use ground beef, many call that cottage pie. However, most people these days use the meat that they love and is available to them. Ground beef, ground pork, ground lamb, or ground turkey work well. I find that ground beef works best. Ground lamb also works well. If you're going to use ground turkey or ground pork, try not to use meat that is too lean, or the dish may end up dry and lacking flavor.