Final answer:
The bison was the most critical animal resource for Great Plains Indians, with its meat and hides providing sustenance and material for clothing and shelter. The introduction of the horse dramatically changed their lifestyle, enabling them to become effective nomadic hunters of bison herds.
Step-by-step explanation:
The most important animal resource for the Great Plains Indians was the bison. This immense creature provided the Plains tribes with meat for sustenance and hides for making clothing, shelter, and other necessities. Before the reintroduction of horses by Europeans, Plains Indians like the Blackfoot relied on dog transport and walking to follow the seasonal movements of the bison. The horse significantly changed their way of life, allowing them to become nomadic hunters who could more effectively track and hunt the bison herds. Other animal resources included deer, sheep, antelope, and smaller animals such as rabbits and birds, which were also used for food and clothing. Plant resources like berries, nuts, and seeds supplemented their diet.