SIRIS labeled this photo "Four Bears", Dakota, but I think it is more likely Red Feather, Sans Arc. According to the Smithsonian it was taken by photographers Hamilton & Hoyt of Sioux City.

This picture is from a series of delegation photographs taken in the early 1870s (1871?). The delegates were from Cheyenne River Reservation. The other photos I am aware of this series are of Charger (aka Martin Charger, Sans Arc), Four Bears (a/k/a Joseph Four Bears, Two Kettle) and White Swan (a/k/a Little White Swan, Minikoju).

There was a Red Feather, Wiyaka-luta, who signed the Treaty of 1876 among other known Sans Arcs like Crow Feather, Charger and Yellow Hawk. It could be him in the photograph. — Dietmar Schulte-Möhring