The Mustanger by Andy Thomas
A hard riding cowboy is trailing a herd of wild mustangs into a trap in a narrow canyon in the Southwest. As soon as the horses pass the hidden gate, the rider will lock them in.
Wild horses were extremely wary. One method of catching them was to run the herd for several days, keeping them away from their watering holes. The men would use grain fed horses and would change mounts often to keep up with the herd. With the mustangs well worn down, they would driven toward an elaborate funnel-type trap or, better yet, a narrow canyon that could be blocked off.
Sometimes the cagey Mustang stallion would smell a trap, and days of hard work would be wasted.