Samurai required foods that were portable and high in calories for battle. Zoni is one such food. Samurai made zoni with mochi (Japanese rice cakes), dried foods and whatever local meats and vegetables were available.
Modern zoni soups retain this flexible nature — practically every prefecture in Japan has its own regional variation. Some use mochi, others use tofu. Some use a miso soup base, others use shoyu (soya sauce). A wide variety of meats and vegetables are used — depending upon the regional variation.