Mountain goats run through the Needles Eye Tunnel, a popular place for visitors to see mountain goats in South Dakota's Custer State Park. In recent years, tour guides have reported fewer sightings in the area. (Photo by Nicole Schlabach/for South Dakota Searchlight)Daniel Milks, the founder and lead guide at My XO Adventures, has introduced many visitors in South Dakota’s Black Hills to mountain goats. “It’s an eye-opener, and a jaw-dropper,” h…