Seven Chaos Monsters

The Shadow Chaos Monsters are evil beings created by the overuse and built-up negative energies of the Chaos Emeralds over the course of the Sonic series. Currently, the Only Known Chaos Monster is Nazo, but the epilogue of Sonic: Nazo Unleashed ReMIX confirms that there should be seven in total, with one of them being defeated off-screen by Rouge the Bat, E-123 Omega and Lieutenant Griffith.

Concept
Initially, the Chaos Monsters lacked any type of form, and all resembled the Chao God Chaos. However, ever since the Emeralds were used by both Dr. Eggman and Sonic, once freed, the Chaos Monsters choose to take forms of a Mobian (Ergo why Nazo is a Hedgehog, taking his appearance after Super Sonic).

List of the Chaos Monsters

 * Nazo: His name derives from the Japanese word for "Mystery". He took on the form of a Hedgehog Based off of the image of Super Sonic. Nazo is considered the unofficial leader of the Chaos Monsters, though he is also the only one unaware of his Kin. Nazo holds the honor of survived being sent back into the emeralds, and has also turned Super and beyond.
 * Mugo: His name derives from the Japanese word for "Disaster". Currently it is unknown what form this Chaos Monster took, but it is known that he was defeated and Captured by G.U.N., thanks to the Help of Rouge, Omega & Lt. Griffith.
 * Niku: "Hatred"