At the end of the movie, two men each fire their six shot pistols into the air to scare the horse. However, 22 shots are seen and/or heard. They were riding horses at speed and didn't have time to reload.
When both Rockwell and Aldis work with Black Horse, it is always rearing up, but they are clearly making a gesture that the horse is trained to rear up to. They always raise their arms just a second before he rears up, clearly controlling the horse with their gesture. The gesture is made to look like a defensive response to the horse rearing (like they are shielding themselves from the horse's hooves or something), but the gesture clearly happens before the horse rears, not after.
About an hour into the movie when three of the actors are chasing the black horse across a prairie, 2 tall radio towers can be seen in the distance quite distinctly. This movie predates radio and television by several decades.