What Could Have Been: As mentioned on the main page, the film was originally going to have actual aliens.
Also, some Happy Feet storybooks and posters show Mumble as a fully-fledged Emperor Penguin. This is because until late in the film's production, it was intended for Mumble to lose his down feathers during his pursuit of the fishing fleet. However, it was decided that Mumble would keep half of his juvenile down throughout the story, to make him different to the other Emperor Penguins. Damn it, George Miller. Here are a few images of adult Mumble: