Mighty transforming into Mighty-Zilla
You may be looking for the prime zone version of Mighty.
Mighty-Zilla is an alternate version of Mighty the Armadillo from an alternate version of Mobius. In this universe, Chaos Emerald radiation transformed Mighty into a hideous, gigantic monster that threatened the people of Mayor Kintobor's city. Mighty-Zilla was not seen fighting Sonic with the others. (SSS: #12)