hobby horse

Short definition from Wiktionary

Noun

1. 1
   The Irish hobby, an extinct breed of horse.
2. 2
   A child's toy consisting of a (usually wooden or cloth) horse's head mounted on a stick.
3. 3
   A topic about which someone loves to talk at great length.
4. 4
   A puppet-like costume used in some traditional dances and festivals that gives its wearer the appearance of a man on horseback.
5. 5
   An early bicycle with no pedals or brakes.
6. 6
   Alternative spelling of hobby horse.

Verb

1. 1
   to pitch back and forth