On 13w21a, You can't control horses, mules, or donkeys.
Linked issues
duplicates 1
Comments 2
This can be resolved by getting off a horse, taking the saddle off, and then hitting the horse, then applying the saddle via right click. The horse should be movable now
Duplicate of MC-16182, please use the search function to see if your bug has already been submitted. Currently over 55% of tickets are being closed as duplicate.