We never know what we're going to see when we go out for a ride. Near the Green River, we came up on a herd of pronghorn (Antilocapra americana).
They stared at us for awhile, wondering what kind of animal we were. Though these guys didn't seem interested in running, the pronghorn (also called a pronhorn antelope, or simply an antelope, although it actually isn't of the antelope family) can run up to 60mph and cover close to 8 yards in a single stride. Wikipedia cites the pronghorn as the second fastest land animal, second only to the cheetah. They have large eyes with a 320 degree field of vision.
After a few minutes of keeping a close watch on horses and riders, they moved on, in no particular hurry.