The correct answer to this open question is the following.
Although there are no options attached, we can say the following.
This animal was the most significant animal to many American Indian nations of the great plains and its demise was connected to the demise of the American Indian people of the great plains.
We are talking about the Bison, better known in the United States as the American Buffalo that was key for the survival of the Native American Indians of the plains.
Bison were key in the life of American Indians because the Natives used to hunt Bison to feed the families. Bison's meat was rich in protein, much needed by the people to live and resist the difficult climate conditions of the time. Furthermore, Indians took the fur of the Bison used it as pieces of clothes to keep them warm during winter times.