Sonic the Werehog (known by some fans as "Werewolf Sonic" or "WereSonic") is Sonic's beastial form in the game Sonic Unleashed. Sonic the Hedgehog was transformed when Dr. Eggman used a machine to painfully suck out the power of the Chaos Emeralds in its entirety from Super Sonic, with which Eggman would fire a specialized laser weapon at the planet, releasing and awakening the creature known as Dark Gaia from the core of the planet. It is stated in-game that it is Dark Gaia's power that causes Sonic's nightly transformation into this form.
Sonic transforms into the Werehog at night, which makes his quest more complicated. When transforming into the Werehog (or transforming back into normal), a cloud of purple smoke envelopes him. Sonic the Werehog also features a different gameplay than Sonic. He attacks with his arms and fists, which can stretch out and punch his foes. Rather than using a counted number of rings, Sonic uses a health bar, though collecting rings refills the bar. Aside from the usual ability of Sonic being able to race down ramps throughout the levels, in Werehog form you must get across gaps in a tightrope walking fashion.
Sonic appears to retain most of his normal abilities in this form as well as the ability to speak, though with more of a growl than normal. Therefore, it can basically just be considered a darker, feral form of Sonic's rather than as a split-personality monster like other interpretations of werewolves. Sonic the Werehog still has Sonic the Hedgehog's good heart, even though he is in a wolf-like form. This is primarily due to his will; it's too strong for Dark Gaia to affect.
In the mini-movie Sonic: Night of the Werehog, Sonic the Werehog unknowingly gained the affection of a female ghost that two male ghosts were trying to impress.
Before the battle with Dark Gaia, Dark Gaia reabsorbed its remaining power that Sonic had incidentally taken at the start of the game, returning Sonic to his default form. It is unknown if Sonic the Werehog will appear in future games.