Though it's definitely inspired by some of the Eastern influence on '80s popular culture, the Teenage Mutant Ninja Turtles franchise is uniquely American. Taking the States by storm in the '80s and '90s, the acclaim of the Ninja Turtles eventually took them to the Land of the Rising Sun, where they experienced perhaps their most bizarre mutation yet.
Mutant Turtles: Superman Legend was a fairly bizarre Japanese OVA that took the Ninja Turtles and filtered them through all manner of anime tropes. Capping off the end of the Turtles' first phase of massive popularity, yet it's still an incredibly obscure product even among the fandom.
Mutant Turtles: Superman Legend
Though its name would suggest a connection to a completely different franchise, Mutant Turtle: Superman Legend was an anime adaptation of the by then already iconic terrapin quarter. Coming out in 1996, the OVA was released in the same year as the final season of the original 1980s' Teenage Mutant Ninja Turtles cartoon that pushed the franchise into stardom. The two-episode special also featured most of the cast of the Japanese dub of the '80s cartoon.
The series is tentatively in continuity with the original cartoon, featuring similar designs and characterizations, though it takes things in a wildly different direction. The plot involves the Turtles finding the MutaStones in a shrine created by the Neutrinos, with the gems allowing them to transform into the superheroic Super Turtles. These forms are much more humanoid in appearance, essentially just looking like humans with green skin.
Unfortunately, they're not the only ones who gain access to MutaStones, as the villainous Krang, Bebopy, Rocksteady and Shredder are there, too. The formerly human animals become more powerful versions of themselves, while the Shredder turns into the winged, dragon-esque Devil Shredder.
TMNT: Metal Mutants
The comparisons to the Super Sentai/Power Rangers franchises were rather obvious, with the Turtles looking like Rangers in a TMNT skin. This is only made stronger by the fact that the Turtles can cross their arms and combine into the powerful Turtle Saint. This winged warrior was a sort of biological Megazord equivalent, and he looks even less like a turtle than the Super Turtles.
This was only featured in the first episode of the two-part OVA, with the second episode having concepts that basically combined Transformers and Saint Seiya. It introduced the Metal Beasts, which were animalistic armors that allowed the Turtles to transform into animal forms. The corresponding Mutanite stones were also elemental in nature, providing a variety of powers of their own.
The series was capped with an excellently wacky theme song sung by anime intro crooner Hinorobu Kageyama, who's most notable for themes in the Dragon Ball franchise. Due to its never being released outside of Japan, the OVA is incredibly obscure, even among the most diehard Ninja Turtles fans. Its most notable appearance was a cameo glimpse in the animated Turtles Forever, which did at least stoke more interest in the bizarre take on the franchise. So far, however, the Mutant Turtles OVA has remained a mere Superman Legend.