Hobby-horse

A hobby-horse is child's toy horse popular during the days before cars. Just as children today will imitate the grown-ups driving cars so, in former times, the children played at riding up and down on a wooden hobby-horse. A larger hobby-horse is also a feature of Mummers Plays and the Morris dance in the British Isles.

By extension, the term also means an obsession of which a person won't let go. For instance, if a person keeps on and on about income taxes being too high someone might say "oh, he's on his hobby-horse again..."