Concept art shows that at one point Mighty was taller and skinnier than Sonic.
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Mighty, on the other hand, was seemingly undone by Takashi Thomas Yuda inventing Knuckles to be a minor one-shot antagonistic rival character, only for the character to become a fan-favourite that appeared throughout the series more frequently than anticipated.
However, internal discussions and a company-wide submission contest ended up with Tails usurping Ray's spot as Sonic's number one sidekick. Ray and Mighty were positioned to be part of the series main cast by character designer Manabu Kusunoki. A lot of this may have to do with unexpected reactions in the audience to certain characters. He later returned after a twenty-three year long absence in Sonic Mania. Lampshaded by his missing poster in Sonic Generations. Chuck Cunningham Syndrome: Has not made an appearance in the franchise since at least Knuckles Chaotix.Knuckles Chaotix being supposedly deemed non-canon (which has since been reversed) may be why said poster says "Missing since 1993" rather than "Missing since 1995". The Cameo: On a missing poster in Sonic Generations.The Bus Came Back: Mighty makes a long-overdue comeback as a playable character in the Updated Re-release of Sonic Mania.Beware the Nice Ones: As mentioned above, threatening his friends is a very easy way to get the otherwise peaceful Mighty to show you where he gets his name.Berserk Button: Harming or threatening Ray, as seen in Sonic Mania Adventures.While he's spinning, he's also immune to spiked objects (he'll either bounce off them or hit them and absorb the damage without actually losing rings or his shield). This can be done either when he's crouched or spinning.