As Meledandri said, in order to make The Lorax a feature-length film, it was necessary to create a world and characters outside of what existed in Seuss's pages. Happily, some of these additions include some adorable new woodland creatures.
"The challenge is that the stories tend to be short. So, you know, how do you approach that? Well what we've always done is we've looked at, well, what's happened before page one? What happens in between the pages? And what happens afterward? Those are the places that we look to. But with an idea that, at the center of the movie, is still what he wrote."