Yōkai

Yōkai (妖怪) are a class of supernatural or paranormal being in Japanese culture, myth, and religions. Species and groupings of yōkai are massively diverse, and run from beasts to demons to spirits. Their motives, appearances, and abilities are just as diverse.

Since many of the characters in TMNT are from and/or visit Japan, yōkai feature occasionally in various incarnations.

Meta

 * The Ninja Turtles themselves are mistaken for kappa in multiple incarnations, usually by the people of feudal Japan.

2003 TV series

 * Tengu was a catch-all term for Japanese demons/yōkai in this series. Among the tengu mentioned were Ultimate Ninja and [[Daimyo (Battle Nexus)|his father, the original Shredder and his, the Kappa Tengu (which are kappa), and the rat-like Nezumi Tengu.
 * Baku is modeled after the original, chimeric creature from Japanese myth of that name.

2012 video games

 * Many of the Power Rangers villains that appear in Ultimate Hero Clash 2 are based on Yōkai: Cat O'Clock is a nekomata, Ripperat is a kamaitachi, Skullgators are alligator gashadokuro, and Stonedozer is a construction equipment-themed diadarabotchi.

Rise of the TMNT

 * Big Mama is classified as a yōkai on TMNT's Instagram page. She is likely based on a jorōgumo, as Joroguma before her was.
 * Baron Draxum is also classified as a yōkai on that Instagram page, but he has also been referred to before and since as a mutant.