MLA

Big Cow. Arapahoe Chief. William Soule, 0AD. https://collections.library.yale.edu/catalog/2010353.

APA, 6th edition

Big Cow. Arapahoe chief. (0 C.E.). [Photographic prints, Photographs, Studio portraits]. William Soule. https://collections.library.yale.edu/catalog/2010353.