From Sonic - Wiki on Neoseeker

[edit] History

Mighty the Armadillo was a member of Chaotix. He was friends with Knuckles The Echidna, Vector the Crocodile, Charmy Bee and Espio the Chameleon. Mighty did not make an appearance in Sonic Heroes, possibly due to the fact that his owner left SEGA and took all copyrights with him. However, Mighty is still a character in the Sonic the Hedgehog Archie Comics.

[edit] Trivia

It was thought by many fans that Mighty was in fact Sonic, due to their similar shape and general appearance.