You show me where this is defined somewhere in a dictionary and I will happily work within that definition.
To me, rare = your definition of unicorn. A rare car is a rare car. Hard/impossible to find, not likely to see another one.
I was at that car show with the XJ220. The owner opened the door for me. I smelled the leather, touched the door panel, and as a car geek, it was quite an experience...one I will never forget, as I am not likely to even SEE an XJ220 in person again.
It's a RARE car. Unicorn? Not to me.