Which Sonic game introduced the Werehog form?

The Sonic game that introduced the Werehog form was Sonic Unleashed.

Released worldwide in 2008, Sonic Unleashed follows Sonic after Doctor Eggman uses the Chaos Emeralds to fire the Chaos Energy Cannon. The blast awakens the ancient entity Dark Gaia, fractures the planet into pieces, and unexpectedly transforms Sonic into the Werehog at night. The form resembles a werewolf and gives Sonic greater strength, longer reach, and stretchable arms, but removes much of his familiar speed.

The game divides its action between fast daytime stages and slower nighttime stages focused on combat, exploration, and puzzles. Sonic Team created the Werehog as a way to introduce a different style of play and broaden the game's appeal. The story also introduces Chip, a friendly creature whose forgotten identity becomes important to the plot.

Sonic Forces and Sonic Colors feature different gameplay ideas but do not introduce the Werehog. Sonic and the Black Knight changes Sonic's setting and equipment, placing him in an Arthurian fantasy world rather than giving him this transformation.