The leaked poster that revealed what is now known as Mega Mewtwo Y apparently referred to it as 'Éclair Forme' Mewtwo when it first popped up on image boards. I know it was probably supposed to refer to the French word for lightning (y’know, as if this region wasn’t French enough without Mewtwo going OH HOH HOH on us), but… Éclair.
And that explains the new forme: he just couldn’t keep the creamy filling in his genes.