This is my attempt to design a LEGO® compatible unicycle or rather giraffe, because unicycles aren’t possible due to the space between the minifigure’s legs being too small.
It needed some improvements (smaller saddle etc.) after the 3D print, but you can see images of the improved version at Shapeways.
I used the original wheel of a LEGO® bicycle.
Obviously, I had to cheat a bit for the second image, because the guy’s balancing skills aren’t that great.