Photo of a Vintage One Wheel Motor Cycle ala Men in Black III
When I saw the movie MIB III and the young MIB people past version of K had a one-wheeled motorcycle that the present-day J had no idea had ever existed I thought it was pretty silly.
But, wouldn't you know ... in 1931 there really was a one-wheeled motorcycle. I think I know why it didn't last! But, it is an interesting idea.