I don't think any live thing is supposed to be riding in a "trailered" vehicle.
I think that only applies to people. I mean when I was kid we brought home some calves in the back of my dads truck and had a foal in the van once to go to the vet. After that my family bought a livestock trailer. Some people feel their pets are family as do I but the laws are the same for pets as for livestock.
That being said I have seen some pretty neat pet transport trailers. There is no way my Doberman would ever get in one to to travel though.