The cat organ, or Katzenklavier as it is known in German, is a fictional instrument that has been brought to life by British sound sculptor, Henry Dagg. In 2010, Dagg famously entertained The Prince of Wales and the Duchess of Cornwall with his plush pussycat piano during a Royal garden party, where Prince Charles broke down into a fit of laughter upon hearing the instrument, and Camilla required a hanky to compose herself during the unusual musical purrformance.
According to Wikipedia (yes, the cat organ has its own Wikipedia page), the inspiration behind Dagg's device is actually a rather cruel invention, "consisting of a line of cats fixed in place with their tails stretched out underneath a keyboard so that cats cry out in pain when a key is pressed. The cats would be arranged according to the natural tone of their voices."
Thankfully, there is no official record of a "real" Cat Organ actually being built, but it has been described in various pieces of literature, most famously by French writer Jean-Baptiste Weckerlin in his book "Musiciana."
In 2009, Nick Cave narrated an animated short from Australia called "The Cat Piano." The award-winning clip tells the story - in the form of a poem - of a cat who must save all of the kitties in his city after they have been catnapped and forced to become cogs in one dastardly human's torturous musical device.
If you didn't know, now you know!
Thanks to Dan and Megan for the tip!