It might be funny to you, and for YOU, there was no issue with the horse's owner, but BreakableRider, and myself, would be beyond furious. It's like picking up the wrong kid at daycare and taking it for a snowcone, or sleeping with another woman's husband. if you think that is an exaggeration, it's not. If I went to the barn and found someone on my horse, I would lose my mind completely. I hate when folk hang on the fences and try to feed or pet my horse, never mind ride her. Horses can be very personal creatures for some people, not just commodities or lesson animals. My horse trusts me to keep her safe and I take that job very seriously. If something happened, even a fright, because someone took her out without permission, I'd feel so responsible. If *I* took someone else's horse out by accident, I wouldn't be laughing about it; I'd be deeply apologetic.
Granted, in your situation, you have the leisure to be amused, but Roman seems to think BreakableRider overreacted, and I'm just here to say she was being kind in her response.