A man’s favourite amusement, or study, is called his hobby horse. It also means a particular kind of small Irish horse: and also a wooden one, such as is given to children.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Hobby * Hobby HorsicalNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes