Hobbyhorse

noun

1. The Irish hobby, an extinct breed of horse.
2. A child's toy consisting of a (usually wooden or cloth) horse's head mounted on a stick.
3. A topic about which someone loves to talk at great length.

   "Don't ask him about global warming: it's his favourite hobby horse, you'll never get away."

4. A puppet-like costume used in some traditional dances and festivals that gives its wearer the appearance of a man on horseback.
5. An early bicycle with no pedals or brakes; a draisienne.