Originally, Danbo was a cardboard box robot from the manga Yotsuba&!.
But most people recognize Danbo from a photo-shot (that's how I knew of Danbo before I finally read the manga) and if you try to google image search it, the result will be something like this, not the Yotsuba&! manga.
So how could Danbo become so popular? Is there someone who used it for advertising or something?