I believe that only Regigias, the Weather Trio, the Lake Trio, the Creation Trio, Arceus, The Kami Trio and the Tao Trio are all unique. The other legendaries can breed, but just choose not to for the player. Mewtwo, Regirock, Regice, Registeel and Genesect were created, so it's possible that more could be made.
'When thou enterest into Oblivion, Oblivion entereth into thee.' -- Nai Tyrol-Llar