Super Princess Peach is a platform game developed by Nintendo SPD and TOSE and published by Nintendo for the Nintendo DS in 2005 / 2006. In the gallery below you can see an early version of the Princess Peach, when it almost looked like a GBA game. It’s possible that the project started on the GBA, but it was later ported to the “new” console.
The Koopalings were originally going to be in the game, but were taken out for unknown reasons. All of the Koopalings’ lost sprites have been discovered by now. Oddly enough, Morton’s sprites were spread around the internet long before those of his siblings. Also, Roy was given a green shell instead of a pink shell. This may have been either a design change or a mistake.
Lost sprites of Glad and Calm Goombas have also recently been discovered. In the game, only Mad and Sad variants appeared. The Glad Goomba (which was a bronze or yellow color), based on its sprite, could be able to bounce or dance around (as the glossary says most Glad Enemies do), and the Calm one (which was green and rather sickly-looking), based on the Calm Enemies, it would probably be sleeping and act like a normal Goomba when Peach is too fast.
A very early screenshot shows the “Peach’s face” screen, which ended up on the bottom screen, to be on the top screen and totally devoid of Heart Panels and also having a bright green, solid background and a different design for Peach. The fact that the Heart Panels, which were used to change Vibes in the final game, are lacking from this screenshot suggests that Super Princess Peach was once planned to not use Vibes and be a more average side-scroller. [Info from Mario Wiki]
Ragey from RandomHooHaas.com managed to make a cheat to warp to some test levels, but it would only work using an emulator with Cheat Engine, an external hacking program, attached to it. Goomther managed to get there using the same method, and even provided video footage of the levels in action.