A few notes about horses in the Gilded Age and Progressive Era I learned in J. Edward De Steiguer's Wild Horses of the West: History and Politics of America's Mustangs (Tucson: University of Arizona Press, 2011).
During the mid-19th century the wild horse population numbered about 2.5 million animals. They descended from Spanish horses that escaped the missions in the 17th and 18th Century either on their own or with the aid of Native Americans. Combined with millions of buffalo and cattle, the hoofed animals did significant damage to the fragile ecosystem. During the Gilded Age their range shrunk as cattle ranches and farms moved further west. One of the forces protecting the wild horses was the great open breeding programs. Cowboys preferred to send their horses out to the wild herds to breed. But a series of disasters decreased the demand for new horses. Although De Steiguer does not mention it, I think the Blizzards of 1886-88 had to have had some effect on the horse population. Not so much from the snow itself, but the effect it had on the cattle industry and thus the need for mounts. De Steiguer does attribute a decline in horse demand to the depression of 1893 and the automobile, which seem reasonable enough. What remained of the wild horses were sold off in several large shipments, mainly to the British Army during the Boer War and later during World War I. Those animals not shipped off to war, fared no better. Although the total horse population in American reached its zenith in 1920 at 20 million head, the wild percentage of the total would shrink even more dramatically. As prosperity spread, people purchased homes and pets, wild horses were round up and slaughtered for pet food and glue, made into baseball covers, and canned for food to be sold in overseas markets. The Taylor Grazing Act of 1934 with its fencing provisions vastly compounded the ill fortunes of the wild horse. By channelling horses off of farm land and funneling them into an increasingly narrow area, they became easier prey for the slaughterers. By 1940 they were about gone.