Searching Google images, this happened to catch my eye: it's Dave Hershberger's entry in the 2005 Kinetic Sculpture Race in Baltimore, a 42-mile trek made by an assortment of bizarre vehicles. Aptly dubbed Unwheeldy, it's a two-wheel hand-made bike with nine-foot wheels, 112-tooth sprockets, 120 spokes per wheel,and--oh yeah--it floats. (Here's Hershberger's journal of its construction.)
Surely, thought I, this is an oddity, a one-of-a-kind.
Not really. There's the Killer Tomato, and a slew of historic side-by-side two-wheelers called dicycles:
And don't forget the monowheels: