What is the meaning of Hobby_horse?

The Irish Hobby, an extinct breed of horse.

A child's toy consisting of a (usually wooden or cloth) horse's head mounted on a stick.

A topic that elicits a prolonged and enthusiastic discussion from an individual.

A puppetlike 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.

To pitch back and forth.

Source: wiktionary.org