What is that gun? It looks like the Beretta Nano, but a lot of parts don't really match up...
Edit: Some people on the artist's twitter are calling it the Double Derringer. That's probably what it is.
It doesn't resemble a Remington 95 (the canonical "double Derringer") very much at all; could be a Semmerling LM4, drawn by someone who didn't have a reference photo handy and only sort of remembered what it looked like. I suspect it may be an entirely fanciful Small Gun. :)