Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
hobbyhorse [ˈhɔbɪhɔːs] n (fig) люби́мый конёк#
he is on his hobbyhorse он сел на своего́ люби́мого конька́
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.
Русский: