A hobby horse (or hobby-horse) is a child’s toy horse, particularly popular during the days before cars. Just as children today imitate adults driving cars, so, in former times, children played at riding a wooden hobby-horse made of a straight stick with a small horse’s head (of wood or stuffed fabric), and perhaps reins, attached to one end. The bottom end of the stick sometimes had a small wheel or wheels attached.
Encyclopaedia Britannica (11th ed., vol. 7) (New York, NY: The Encyclopaedia Britannica Company, 1910)