Unicycles are objects used for the unicycle race in "Rescission". They are very unstable, thus difficult to ride on, as shown in the episode. Leafy had gotten used to it, and Bubble won unintentionally without knowing how to ride a unicycle. They are apparently flammable, as Firey burned directly through one.
Unicycles are bikes with only one wheel. It does not have supporting parts to hold on, only the seat and the wheel. Unlike their real life counterparts, these unicycles do not have pedals.