Kamek

Kamek is a Magikoopa in the Super Mario Bros. series that appears as a minor antagonist. They usually appear as a regular Magikoopa, though tend to be a boss fight instead of a regular enemy. They are one of the 2 main antagonists of Yoshi's Island, paired with Bowser Jr.

Mario Kart 64 Manifestation
Kamek was originally fully implemented in Mario Kart 64 as a playable driver, though were later removed and replaced with Donkey Kong. Why they were removed exactly is unknown, though the main theory is that they were considered too obscure. They have been theorized to be an unlockable character, though this is untrue.

Super Mario 64 Manifestation
Kamek can supposedly appear in Super Mario 64, likely due to the Personalization A.I. They usually appear as a boss fight, acting the same as a regular Magikoopa, but on rare occasions have been a playable character.