Making a robot that walks on legs is a much tougher challenge than making a robot that gets around on wheels, and when you add in the weight of a human rider, the job becomes harder still.
This enthusiast-built Russian ostrich robot apparently uses a sliding internal weight to keep its balance, but it still looks like you need to move with caution. In the video clip below (from a Russian TV show) you can see the ostrich walking, although it's actually a dummy and not a real human riding on its back. Later in the clip you can see a human climbing on with great care, but he doesn't actually take it for a walk.
Still, this is a pretty impressive achievement for a bunch of guys with limited resources. The group includes an electronics engineer and a an aviation tech, but no robotics professionals. If they can put this together with a bunch of wood and a few motors, it makes you wonder what they could create with some real funding.