hobby horse 
noun
1A child’s toy consisting of a stick with a model of a horse’s head at one end.In the contest, they must race 100 metres with a hobby horse while hopping over jumps....
- Her unicorn hobby horse was a very special gift from Daddy.
- This painting of a child with a hobby horse dates from 1836
1.1A rocking horse.A drawing of King William at the top of this broadsheet, rocking on a wooden hobby horse, might have caused some offense in loyalist circles....
- He held her hand and rocked it back and forth like a child on a hobby horse.
- One shot of a young boy claiming to be an expert horseman shows a toddler rocking on a hobby horse.
1.2A model of a horse or a horse’s head, typically of wicker, used in morris dancing or pantomime.Morris men, lords and ladies and even Hobnob the hobby horse were to be found mingling among the stalls....
- The vibrant costumes and scenery gave bright splashes of cartoonish colour to the scenes and the props, such as hobby horses with nodding heads, were superb.
- The Peace Police on their hobby horses added humour and colour to the events.
2A preoccupation or favourite topic: Brennan admits that the greenhouse effect is a hobby horse of his...
- Warning the government to be prudent, and to remain prudent, may sound like my favourite hobby horse, - and it is, but there are sound reasons why I always carry on about not overshooting the main budget.
- It deals with one of my favourite hobby horses, Complexity Theory.
- The other omitted item, and I admit this is a bit of a personal hobby horse of mine, is that there is no mention of the Little Ice Age, though consequences for it abounding in these pages.
- semiochemical
- semi-official
- semi-officially
- semiological
- semiologist
- semiology
- semi-opaque
- semi-opera
- semiosis
- semiotic
- semiotical
- semiotically
- semiotician
- semiotics
- Semipalatinsk
- semipalmated
- semi-Pelagian
- semi-Pelagianism
- semi-permanent
- semi-permanently
- semipermeable
- semi-precious
- semiprivate
- semi-pro
- semi-professional