Merriam-Webster Collegiate Dictionary - hobbyhorse

 noun  Etymology: hobby small light horse, from Middle English hoby, hobyn, perhaps from Hobbin, nickname for Robert or Robin  Date: circa 1553  1.  a. a figure of a horse fastened about the waist in the morris dance  b. a dancer wearing this figure  2. obsolete buffoon  3.  a. a stick having an imitation horse's head at one end that a child pretends to ride  b. rocking horse  c. a toy horse suspended by springs from a frame  4.  a. a topic to which one constantly reverts  b. hobby II