photos from the XIX century can be astonishing in their clarity and information when comes to horse and rider etc. During the XIX century, when the true art of photography was born, Americans were in the forefront of this art and technology.
Most of the photos are still to be digitalized but many are already on line, accessible to anyone with a Internet access.
Wikimedia Commns has a great gallery - Hayden Survey, William H. Jackson, Photographs, compiled 1869 - 1878 - of the American surveyors and explorers of then Wild West.
William Henry Jackson, also a noted painter ( like the image above and here recalling the Great Plains in his art) and photographer, took these images - there are thousand available - during the Dr. Ferdinand Vandeveer Hayden's expeditions to the Rocky Mountains, including the famous 1871 Yellowstone expedition.
Bellow W. H. Jackson photographed in 1872
explorer of the Yellowstone River and the leader of the numerous expeditions to the area that led to the establishment of the Yellowstone Park in 1872.
From these photos we get a glimpse of horses and equipment used by the the 1870s American explorers and outfitters during the exploration of the Rocky Mountains etc.