Recently a book called The Legend of Zelda: Breath of the Wild Master Works saw the light of day in Japan, and it was perhaps thought that it would be a fairly straightforward artbook with illustrations on the things found in the game. However, a scan of pages from the book appeared on Reddit and Gaming Reivented went through it and uncovered a bunch of secrets.
As is the natural process of game development some ideas and concepts never make it into the final game and in the book, there are six designs of Divine Beasts and 5 Guardians that ultimately didn't make the cut. Of course, the fact that there are concept artwork doesn't necessarily mean Nintendo ever had actual working versions of these concepts up and running in the game as they could have been dismissed at an early stage.
Another interesting bit of information shared by Nintendo staff in the guide is what Hyrule Castle and Castle Town looked like before the Calamity. There are also a lot of sketches of Princess Zelda and many other characters.
The Legend of Zelda Breath of the Wild Master Works has not yet been announced for Europe or America, so, for now, fans have to make do with the Japanese edition.