hobby: c. 1400, hobi, “small, active horse,” short for hobyn (mid-14c.; late 13c. in Anglo-Latin), probably originally a proper name for a horse.

The modern sense of “a favorite pursuit, object, or topic” is from 1816, a shortening of hobbyhorse in this sense, which is attested from 1670s. Earlier it meant “a wooden or wickerwork figure of a horse,” as a child’s toy or a costume in the morris-dance, the connecting notion being “activity that doesn’t go anywhere.”