Boise inventors Ben and Anna Carter have one issued patent on their pet trailer, U.S. patent 8,950,767, and another patent on the folding version of the trailer, 20140216352, is about to issue. The trailer described in these patents attaches to a bike, and has a floor that folds up, so a dog can run inside the trailer for awhile, then the floor can be put down and he can ride. They come in small, medium and large, for all sizes of dogs. Each size holds two dogs, and one dog can ride, while the other runs, both run, or both ride.
My dog quickly figured out what the deal was, and loved it. More information can be viewed at puprunner.com.
Below: the pupRUNNER trailer attached to my recumbent trike.