The Mughals introduced Nalli Nihari to the Indian subcontinent. Initially, Mutton Nihari was favored by the noble lot, however, over the years other types of meats such as beef, buff, etc. were introduced. In Pakistan, Nihari is mostly cooked with beef. These days, Chicken Nihari is equally popular. In India, you can see different versions of Nihari cooked with Beef, Mutton, or Chicken.
Shank is mainly round thigh meat. The muscles of the upper thigh of the animals due to their bone structure are tough and dry and also full of marrow. This particular type of bone is the source of gelatin. Gelatin is a degraded form of collagen and hours of cooking release the collagen into the stew.