From Google search
Why do they cut the Clydesdales tails?
Docking. Docking traditionally has been performed to prevent the tail of the horse from interfering with harness and carriage equipment. Specifically, if a rein passes under the horse’s tail the horse may clamp its tail down and cause the driver to lose control of the horse.
IOW mutilating animals for man’s convenience.
These animal rights activists need to try cleaning the matted horseshit out of the tail!