hobby horse
noun /ˈhɒbi hɔːs/
  /ˈhɑːbi hɔːrs/
- (sometimes disapproving) a subject that somebody feels strongly about and likes to talk about
- to get on your hobby horse (= talk about your favourite subject)
 
 - a toy made from a long stick that has a horse’s head at one end. Children pretend to ride on it.