There is a type of bicycle that is sometimes used as an intermediate step to riding a unicycle -- it's basically a unicycle for the back wheel of the bike, with a pivoting bar attached to it with a standard front wheel at the end of it.

Or you could describe it as a regular bicycle with a direct-drive, pivoting, rear wheel.

All of which to say that it is a bicycle, and its wheels can make any track you like as long as the front and back tracks are a fixed distance apart.