If I may follow the somewhat broader definition, Henri Mignet's Flying Flea is certainly some kind of unique - at least in concept and design; probably some other variation of unique in its flying characteristics. I knew a bloke once who had one. He claimed it was OK so long as you understood it (whatever that meant - he didn't expand and I didn't ask). Quite sad that he died (reportedly of a heart attack) whilst enjoying one of his local flying outings in the aircraft. I guess that's sort of unique too - in the saddest sense.