After approaching a skeleton trap horse and dealing with the four horsemen that spawned as a result (in a vanilla survival world), I attempted to ride one of the left over skeletal horses. I was able to get on the horse, but even with a saddle I cannot move the horse at all. It will stay stationary when trying to move around, and will only raise it's front legs if an attempt is made to jump. This was tried with all four skeletal horses.