A subject, topic, or issue about which one frequently or incessantly talks, expounds, or complains. Ah, here we go again. Once grandpa gets on his hobby-horse about the government, there's no stopping him!
a ˈhobby horse
a subject that somebody feels strongly about and likes to talk about: Sorry, I’m starting to get on my hobby horse! (= talk about my favourite subject).
There are numerous videos on YouTube about how to make a hobby horse, and the components are easy to find: a broomstick, fabric, fur, wool, stuffing and decorations - which are obviously up to each creator.