On 13w21a, You can't control horses, mules, or donkeys.
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duplicates
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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.