car entirely. If that’s not feasible, consider fitting your dog with a safety harness that can be attached to a seatbelt receptacle---just be sure to keep him from pushing his head out the window. Allowing a dog to ride in the trailer with the horses is unsafe for all involved.
At a show, clinic or similar event, there may be times your dog has to wait at your trailer by himself. Don’t attempt this unless you know your dog will be OK with the temporary separation, and even then, create a safe, secure space for him in the trailer or a crate nearby. Leave him with plenty of water, a comfortable place to lie down and maybe a toy or two. Of course, never leave your dog in a closed vehicle on even a mildly warm day.