The Collaborative International Dictionary of English v.0.48:

Dray \Dray\, n. [AS. dr[ae]ge a dragnet, fr. dragan. ????. See
   Draw, and cf. 2d Drag, 1st Dredge.]
   1. A strong low cart or carriage used for heavy burdens.
      --Addison.
      [1913 Webster]

   2. A kind of sledge or sled. --Halliwell.
      [1913 Webster]

   Dray cart, a dray.

   Dray horse, a heavy, strong horse used in drawing a dray.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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