The Collaborative International Dictionary of English v.0.48:

Horseless \Horse"less\, a.
   Being without a horse; specif., not requiring a horse; --
   said of certain vehicles in which horse power has been
   replaced by electricity, steam, etc.; as, a horseless
   carriage or truck. It was used primarily in the term
   "horseless carriage", to refer to automobiles. By the 1930's
   when automobiles had become more common than horses for
   transportation, the term had lost its currency. [archaic,
   except in a historical context.]
   [Webster 1913 Suppl. +PJC]