Horse trailer safety and abuse?

I recently encountered a situation on the highway and wanted to know whether or not it was illegal. I called HP who then told me to call 911, and they did not seem concerned. I saw a miniature pony saddled and haltered on the back of a "lawnmower" trailer. The trailer was full of untethered equipment including tools and a riding lawnmower. The pony had enough space to stand and shift his feet. There were no walls to speak off, except for the back loading ramp and a short rails surrounding the trailer. It is 22 degrees outside and we were going at least 65mph. I felt like the responder was indifferent to the situation and didn't take me seriously. This is a rural area with a low crime rate and I'm certain that they do not have more pressing matters at this time, so I don't believe that was the case. If there happened to be an incident on the highway I am certain that the Pony would have been thrown from the trailer, or at least had all this untethered equipment slammed into it. While I feel certain that this is inhumane treatment of an animal I was wondering it was illegal. I also wanted to know if there was anything that I could do in the future if I see a similar situation or something that I believe to be inhumane.