add mantyke and castform.
Castform and Deerling/Sawsbuck are a bit different, I dare say, as their pallette/form changes are entirely due to their environment, such as the weather or season. (on that note, are there seasons in X and Y? I hadnt noticed them. If there arent, how does Deerling's form change work?)
In other form/pallette changes there are also the item variations, like Arceus with Plates, or Giratina in the Distortion World, Rotom after touching items or Shaymin with a Gracidea flower.
Giratina, Rotom and Shaymin are big sprite changes (with Rotom and Shaymin even changing type), but I think Arceus is just a colour change, and so is Genesect too.