When you are riding a tamed horse, if you use a sword on it, its durability wears down and the horse suffers no damage. I assume that you're not supposed to be able to hit your horse as you ride it.
Linked issues
duplicates 1
Comments 5
Duplicate of MC-4732.
note: the horse will not take damage if you try hitting it with a sword but de sword does get damaged!!!
MC-4732: Tools and weapons lose durability without doing damage
Confirmed, you can test this by riding a horse and hitting it with a gold sword, the sword will lose durability when the horse is hit.