Sonic had many defferent forms in the series. Currently Sonic shown 5 out of the 9 form planned for the Sonic Storm Adventure Series
Sonic's forms
Super Sonic
The common Sonic Form, Super Sonic
The most common of Sonic's transformations.
Super Sonic is a super transformation and powered up version of Sonic the Hedgehog that can be activated through collecting all seven Chaos Emeralds and usually fifty rings. In the classic 2D games, he plays like Sonic but can jump higher, is faster, and is invincible. Nevertheless, he could still drown, die by falling off screen or get crushed against a wall.
It made it's first appearance in Sonic Storm Adventures of Kingdom Hearts.
Darkspine Sonic
A rare transformation of Sonic, since it only be awakened by the 7 world rings.
Sonic's Head-Beast forms
The side effect of being unable to control Orochi is that when Sonic
Death Sonic
Death went inside Sonic and became Death Sonic, but Nancy got rid of Death once and for all.
Werehog
In Sonic Unleashed, when Eggman Relesed Dark Geia (using the negative energy of the Chaos Emeralds and Super Sonic) Sonic was turned into a Werehog as a side affect. Now by nightfall sonic becomes a werehog to fight Dark Geia's minions.
Sonic became a werehog once again in Halloween Town in Sonic Storm Adventures of Kingdom hearts.