As far as I can tell, it originates in a joke about a performer - perhaps a vaudevillian? - whose act involved a marvellous trick-performing pony, which, as it turned out, could only do one trick. (It's related to the one about the guy with the talking dog: "What do you call the thing on top of a building?" "Roof!"
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Date: 2005-09-17 03:48 pm (UTC)