Here is the original Giratina in full 3D on the Platinum beta title screen, similar to Dialga and Palkia for Diamond/Pearl

The used model in the final is of the new form
The scratchpad for Gen 3 brings about even more revelations for not just the process of creating the individual sprites, but also for alternative designs of the Pokemon that made it to the final build of the game.
First, the boring stuff. Some overall observation that can be deduced from the existing sprites:
*The transparency color was probably white for Pokemon originally
*The canvas for sprites used a smaller 56x56 size compared to the final 64x64
*Many older Pokemon use either the Gen 1 official art or Gold/Silver sprites for reference
*Gen 2 Pokemon were either done first or had least amount of revisions
*Animations originally looked more like they did in Crystal compared to the final release
Now for the interesting stuff, the alternative designs for Gen 3 Pokemon. Some of them also show more interactive original animations, probably aimed for the Emerald release.
Way too much to talk about here. I am more surprised that they even went as far as to try to actually recreate the extremely edgy art that was posted around the start of the leak. If you're into spriting, there is a whole bunch more technical details on the TCRF page as well, including leaked beta palettes
Some high quality concept art I found, either it's new or I didn't see it posted yet




Misc XY stuff:
*Mega Jynx was confirmed to be in the game files, as some of you may already know. The cry was leaked even before the Terraleak, altho many thought it was a hoax. Sadly, this form never got any artwork, models or a mega stone, showing how it would be scrapped rather early. It would have had Magic Guard and a massively increased SpA/SpD as well as minor increase to Defense and Speed
*Xerneas was originally a Dark Type and had Pressure Ability, before it got a Fairy makeover
*All three stages of Aegislash would originally have Stance Change Ability
*Sylveon had Synchronize instead of Cute Charm
There is Gen 1 leaks as well, covering every single corner of Pokemon history at this point.
Not much, just some early Yellow sprites...for now
Of note is that a cropped sprite for Magby, a Gen 2 Pokemon, present within the scratchpad for some reason. Gold/Silver and Yellow were being made at the same time so this wouldn't be out of place, but nobody knows what purpose that would serve in Yellow. Maybe bringing some baby Pokemon into the game to promote Gen 2 was planned?