I was playing on my single player survival world, and it was a thunderstorm, so some skeletal horses spawned. After setting off the traps, i left them alone to die in the sun. However checking back on them later the skeletons were being shown as standing on their own, not moving, with their horse seemingly having no rider. However if you got close to the horse the animation of the skeleton would draw its bow and shoot at you, and then an arrow would appear from above the horse and fly towards you. You could not hit the skeletons where they were rendered, neither where they should have been on top of the horse. At one point I died, and this reset the skeleton onto the horse, however it detached again quickly after.