This guy in Bend, Oregon sells a scooter with a harness to connect a dog or two to get them to pull you. Say "Mush."
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And you should see these things in action! There's someone in SE Portland who uses one to exercise his pitbulls, and so early on Saturday mornings, you can see them roar around the neighborhood. The dogs love it, the owners seem to enjoy it, and the unsuspecting have a good story to tell afterwards!