We used a large log once for a mount from which to shoot a toughman target. By changing the direction of the target you can easily vary the difficulty of it. For a right hand shooter, a target directly in front of the horse, or slightly to the right of the horse becomes difficult.
Eventually someone is going to set up a range where you shoot at an animal target, and then after pulling arrows, mount that target to shoot the next target. The whole range could be shot from "horse back". Obviously it would require large durable targets.
I think that Roger and Ron both have found new avatars.