The Collaborative International Dictionary of English v.0.48:

Draft \Draft\, a.
   1. Pertaining to, or used for, drawing or pulling (as
      vehicles, loads, etc.). Same as Draught; as, a draft
      horse.
      [1913 Webster]

   2. Relating to, or characterized by, a draft, or current of
      air. Same as Draught.
      [1913 Webster]

   Note: The forms draft and draught, in the senses above-given,
         are both in approved use.
         [1913 Webster]

   Draft box, Draft engine, Draft horse, Draft net,
   Draft ox, Draft tube. Same as Draught box, Draught
      engine, etc. See under Draught.
      [1913 Webster]

WordNet (r) 3.0 (2006):

draft horse
    n 1: horse adapted for drawing heavy loads [syn: draft horse,
         draught horse, dray horse]