The Collaborative International Dictionary of English v.0.48:

Hobby \Hob"by\, Hobbyhorse \Hob"by*horse`\, n. [OE. hobin a nag,
   OF. hobin hobby; cf. hober to stir, move; prob. of German or
   Scand. origin; cf. Dan. hoppe a mare, dial. Sw. hoppa; perh.
   akin to E. hop to jump.]
   1. A strong, active horse, of a middle size, said to have
      been originally from Ireland; an ambling nag. --Johnson.
      [1913 Webster]

   2. A stick, often with the head or figure of a horse, on
      which boys make believe to ride. [ Usually under the form
      hobbyhorse.]
      [1913 Webster]

   3. A subject or plan upon which one is constantly setting
      off; a favorite and ever-recurring theme of discourse,
      thought, or effort; that which occupies one's attention
      unduly, or to the weariness of others; a ruling passion.
      [Usually under the form hobby.]
      [1913 Webster]

            Not one of them has any hobbyhorse, to use the
            phrase of Sterne.                     --Macaulay.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hobbyhorse
    n 1: a topic to which one constantly reverts; "don't get him
         started on his hobbyhorse"
    2: a child's plaything consisting of an imitation horse mounted
       on rockers; the child straddles it and pretends to ride [syn:
       hobby, hobbyhorse, rocking horse]