I think that is totally stupid that they can just brand a horse with just any old branding iron!!!!! I also think that since he was a rental horse before I got him and he was from south of the border that maybe it was from a ranch down there!!!
If he was brought up from Mexico, there needs to be some record in the form of "traveling papers". Health papers, proof of ownership, etc. You can't just bring a horse across the border. There has to be a paper trail on him and it can't be cheap to do. I would suspect that there was some value to him for a former owner to go through that trouble.