Source : Webster's Revised Unabridged Dictionary (1913)

Horse \Horse\, n. (Student Slang)
      (a) A translation or other illegitimate aid in study or
          examination; -- called also {trot}, {pony}, {Dobbin}.
      (b) Horseplay; tomfoolery.

Dobbin \Dob"bin\, n.
   1. An old jaded horse. --Shak.

   2. Sea gravel mixed with sand. [Prov. Eng.]

Source : WordNet®

dobbin
     n : a quiet plodding workhorse [syn: {farm horse}]