This wonderful video features Nora, the piano-playing cat. Not a synthesizer, but it is a keyboard instrument.
In addition to the simple cuteness of a cat playing piano, I actually found myself listening to music itself. Clearly a lot of major and minor seconds, mostly because they are easy to reach with a single paw, but there is also the strong repeating rhythm. And she seems remarkably consistent over multiple brief “performances.”
I recommend listening to Nora's music without observing the video, as I am now, and you will hear an interesting minimal atonal piece that stands on its own. Many detractors of atonal and free-rhythm music often argue that “their five-year old could do that” or even that their pet could do that, but perhaps the fact that it captures childlike and cat-like innocence is part of the charm such music.