Definition from Wiktionary, the free dictionary
Alternative forms 
- (originally) The Irish hobby, an extinct breed of horse.
- A child's toy consisting of a (usually wooden or cloth) horse mounted on a stick.
- (figuratively) 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.
- A puppet-like costume used in some traditional dances and festivals that gives its wearer the appearance of a man on horseback.
- An early bicycle with no pedals or brakes; a draisienne.
topic about which someone loves to talk