Sonic Unleashed/Fridge

Fridge Brilliance
Possibly, Sonic becomes the Werehog by absorbing the dark energy from the Chaos Emeralds (and a bit of Dark Gaia). Only one other character has ever drained the dark energy from the CE's and that was Chaos who, like the "Werehog" grew sharp teeth, bulkier, and had the ability to stretch. One wonders why they didn't call him Sonic Chaos...
 * Probably because the name Sonic Chaos already belonged to something else.
 * One may also be reminded of Ristar.
 * There's also the fact that some of the Dark Gaia spawns also had stretchy arms, meaning he could've just gotten the ability from absorbing part of Dark Gaia's essence, which only takes effect at night, triggering the involuntary transformation into the werehog.