I am curious about the origin of the expression of a "riding" a "hobby horse" (or "stick horse", as it is in Danish — we have the exact same expression) as an idiom for a 'pet topic' or 'fixed idea'.
I've only found traceback of the word "hobby" to a horse breed widespread in the 15th century, but have not been able to find anything about the origin of the idiomatic expression as such.
A mere guess on my part, possibly far-fetched, is that it might have derived from the hear-say of witches supposedly riding broomsticks (broomsticks have all times undoubtedly served as stick-horses). Possibly during 1700s - J.Swift's Meditation Upon a Broomstick (1701) parodying R.Boyle's contemporary arguments of any given thing reflecting God's relation to man or his soul - which were very popular (one might say a hobby horse) in the household where he presented it.
Also during the 1700s emerged depictions of witches riding their brooms "the wrong way" (ie. like a stick-horse). Similarly F.Goya's "Linda Maestra!" (#68) from his collection Los caprichos (1799) of which many of the pictures may be seen as quite satirical, drawing on metaphoric or idiomatic symbolism. In the same collection broomsticks also appear in (#20) "Ya van desplumados", in which one woman use her broomstick (her "stick-horse"?) to sweep out little men appearing as chickens while another woman, by the way she is holding hers, rather appear to be using it to beat them up.
Is it possible that during the 1700s - possibly facilitated by Swift's 'broomstick meditation' - the "broomstick" might have emerged as an idiomatic expression and a forerunner to the present-day "stick-horse" or "hobby-horse"?
Or does anyone know more specifically documented about the actual origin of this expression?