A page from the manga explaining the shapeshifter copies.
Ah, lastly, there are also the five artbooks self-published by Ryoko Kui, titled “Daydream Hour” vol. 1-5. I’ve posted the occasional page from these, too, but they also contain art unrelated to Dungeon Meshi.
They can be found on the internet archive.
That’s all the “bonus content” I know of.