Somewhere south of the border of the United States, they make an instrument called a Charango (I think) which is a ten string Mandolin sorta thing of which the back is made from an Armadillo Shell.  I've seen three of these in my life, two which belonged to a Baptist Preacher. 

He didn't have them tuned properly so I tuned them like a mando when I met him with the low string tuned to B. 

The closest I'VE seen to a 12 string mandolin would be a half sized 12 string guitar that was made by a company from France...I think the name was Vielette.  I need to check my NAMM bulletin from last year.  It's tuned like a regular guitar but half sized so it has a nice shimmery mando sound.  It is the little brother/sister of another guitar called a Griphon which is a 12 String Baritone guitar.  Both carry a price tag that is prohibitive to folks like myself.

I'll look these things up in my book and post more later.

Luv Ya,Stay Cool, don't party too hard and see you next Fall!

PRH