The name Danbo is a pun on the Japanese word for corrugated cardboard "danboru" (ダンボー). There's also a company in Japan which makes cardboard boxes which has Danbooru in its name.
There have been a few variations on this name in the west, based on different translations from the original manga. Danbo was the original name used in the Yotsuba&! manga, and has also been used in the most recent translated versions for western markets.
The original western translations used the name Cardbo instead, as a literal play on words of the western word "cardboard", but the "correct" name in the eyes of purists is Danbo, keeping the original Japanese pun intact.
~~~ Peace out ~~~