This is an interesting one. The phrase has become widely used since 1980, when Paul Simon released a recording and film with that title. I recall seeing him interviewed (by the BBC DJ Anne Nightingale I believe) in which she asked him about the title, and he gave the impression that it was a just a phrase he had thought up. Certainly, despite its widespread use and citation in various dictionaries now, there is no credible reference that I can find that predates this, and only one at all - a reference to an early 19th century American circus which appears entirely spurious. The album was one of Simon's least successful, and the film totally bombed, so it is not a phrase that gained huge media exposure, and the subject matter of a circus pony is hardly cool for the late 20th century, so if such a now-popular phrase had originated earlier you would expect to see a lot of reference to it prior to 1980, instead of nothing. My guess is therefore that Simon did coin it, and that it's just one of those happy phrases that people can immediately understand. I think part of its appeal is that its both critical but also sympathetic - we feel kind of sorry for the poor animal - after all it's trying its hardest - and it's a good trick.