hobbyhorse comes from hobby (originally hoby 'small horse, pony', a pet-form of the name Robin or Robert) + horse; the figurative 'pet topic' sense derives from the toy hobby-horse one rides repeatedly. The history explains the modern form through the named earlier word or formation, without requiring a separate speculative ancestry. In present English, the noun sense is "a favourite topic one returns to insistently; a toy horse on a stick", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as hobby and horse. Its semantic neighborhood includes obsession, fixation, preoccupation, and fad, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Middle English hobyn and Old English hors.