Make sure it has a very good suspension system. I have built many load carrying trailers for bike and scooter, and when you combine the DR potholes with high speeds, your buddy may be in for a very unpleasant shake.
If you are into a bit of DIY, a single wheel scooter on a swing-arm with at least 3 inches of suspension travel would be perfect for a 50-100 pound K9 friend.
Brad