Is it safe to pull off the road and pet a horse you don't know?

So my ex and I were on vacation and he saw a pretty horse near the road behind a fence. Always oh, so affectionate, he got out of the car, walked up and pet the horse. Suddenly, the horse grabbed my ex’s arm and left it black and blue. My ex kind of chuckled and walked back to the car with a red face and didn’t mention it. Is this a safe thing to do? I think my ex wanted to punch the horse but had had enough of his teeth.