What is "hobby horses"

Usage examples of "hobby horses".

The boys danced riding their hobby horses while the girls danced carrying their dolls.

Yes, a trike, grey and silver, with an odd black hobby horses head set between the handlebars.

Franklin can see that Washington is waiting until the people around the restaurant table have ridden their individual hobby horses as far as they will go.

Let me tell you, it's hard work being the voice of prudence and good sense when you're all being noble and generous and riding your liberal hobby horses recklessly.