noun (n)

• a topic to which one constantly reverts(noun.cognition)

  Example:
  Don't get him started on his hobbyhorse.
  

  source: wordnet30
• a child's plaything consisting of an imitation horse mounted on rockers; the child straddles it and pretends to ride(noun.artifact)

  Synonym:
  hobby, rocking horse
  

  source: wordnet30