The Collaborative International Dictionary of English v.0.48:

Sulky \Sulk"y\, n.; pl. Sulkies. [From Sulky, a.; -- so
   called from the owner's desire of riding alone.]
   A light two-wheeled carriage for a single person.
   [1913 Webster]

   Note: Sulky is used adjectively in the names of several
         agricultural machines drawn by horses to denote that
         the machine is provided with wheels and a seat for the
         driver; as, sulky plow; sulky harrow; sulky rake, etc.
         [1913 Webster]