Early in the development of The Empire Strikes Back, Bespin was identified as Katbrae, and then Kettlebrae, The Gas Planet. An intriguing concept was the entire planet somehow possessed a cloaking device that made it and its floating cities invisible from space. In these early incarnations, the Gas Planet was home to an intelligent species called the Whatnots, which traveled on immense flying whales.
Lucas has long been trying to get these whales into the Star Wars films. The Bespin whales were recycled into Return of the Jedi concept art, originally as a means of conveyance later replaced by Jabba's sail barge and skiff. They were then possible concepts for the Gungan military when developing The Phantom Menace. An incarnation of these whales will finally be seen in Attack of the Clones. (Taken from StarWars.com)
But where will we see them?