A hobby is a regular activity or interest that is undertaken for pleasure, typically done during one's leisure

Leisure

Leisure, or free time, is time spent away from business, work, and domestic chores. It is also the periods of time before or after necessary activities such as eating, sleeping and, where it is compulsory, education....

 time.
A hobby horse

Hobby horse (toy)

A hobby horse is a child's toy horse, particularly popular during the days before cars. Children played at riding a wooden hobby horse made of a straight stick with a small horse's head , and perhaps reins, attached to one end. The bottom end of the stick sometimes had a small wheel or wheels...

is a wooden or wickerwork toy made to be ridden just like a real horse (which was sometimes called a "Hobby"). From this came the expression "to ride one's hobby-horse", meaning "to follow a favorite pastime", and in turn, hobby in the modern sense of recreation.

Hobbies are practiced for interest and enjoyment, rather than financial reward.