(Left) Swiss engineer Mr. Gerdes astride/inside his one-wheel motorcycle, 1931. (Right) The Mono-Wheel is popular for travel between remote Russian villages.
See Chinese Military in modern times enjoying mono-wheels, presumably for balance training, but maybe simply... for fun?
Some outlandish mono-wheel concepts from 1925 "Science & Invention" and "The Electrical Experimenter" in 1918.
all images from darkroastedblend.com where you can find a complete pictorial and written history on mono-wheels in their archives. Enjoy!