hobby horse
noun
University
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 that elicits a prolonged and enthusiastic discussion from an individual.
4
A puppetlike 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.