Born about 1868 or 1869, Amos was the son of Bad Heart Bull (Tȟatȟáŋka Čhaŋtéšiča) and his wife Red Blanket (Tȟašína Lúta Wiŋ). Amos' father was a brother of the headman He Dog and a nephew of the famous Oglala chief Red Cloud. Known as Eagle Bonnet (Waŋblí Wapȟáha) as a young man, Amos grew up living the traditional life of the Oglala Lakota. His family belonged to an Oglala camp known as the Soreback Band. He was eight years old when George Armstrong Custer's column attacked the large Indian village in the Battle of the Little Bighorn in 1875. The Sioux decisively defeated Custer's forces.
At the end of the Great Sioux War of 1876-77, the Bad Heart Bull family surrendered at the Red Cloud Agency on April 18, 1877, several weeks before Crazy Horse. Following the killing of Crazy Horse in September 1877, the family moved with other northern Oglala to the nearby Spotted Tail Agency. The family fled north with other Oglala, eventually joining Sitting Bull in Canada. After a few years, the Bad Heart Bull family probably returned to the U. S. with other Oglala Lakota, who surrendered at Fort Keogh in 1880.
They were transferred to the Standing Rock Reservation in 1881. The following spring, they were sent to join the rest of the Oglala at the Pine Ridge Indian Reservation in present-day South Dakota.