I rehabbed a stallion that had spent 20 years in a stall with a big hole in the middle. He was terrified of shadows. I realized he thought the shadows were holes. So I jumped into a shadow before him. He was so shocked to see I had not fallen in. He walked right thru the shadow. We were friends for the rest of his sweet life.
Yep, sometimes, its the small everyday things that are the scariest to those big strong guys. My Canyon, was always afraid of a pile of gravel at the end of a road we rode daily. To him, it must have been the biggest black bear, he seen.