I've only seen the one that's a banjo head with a mandolin neck/strings (which we used to call a mandobanjo, and which sounded a lot like somebody squeezing a Coke can to make it "pop" in and out--plinka plinka plinka).

Our OT band played a joke on the audience once at a local bluegrass show--all six of us went onstage with banjos and played (in my case, air mandobanjo) until the emcee, who was in on it, threw us off, saying there was a local ordinance against having more than one banjo per band in action at any given time.