Film character designer J.S. Marantz recently showcased concept art for Krang drawn for an unproduced Teenage Mutant Ninja Turtles movie, featuring two different mechanical body designs. He did not specify which film project these designs belonged to.
After transitioning from comics to the big screen, the earliest live-action Teenage Mutant Ninja Turtles film series released three movies, followed by an associated animated film. A later live-action series released two films, succeeded by the animated movie Teenage Mutant Ninja Turtles: Mutant Mayhem. An adult-oriented film project titled "The Last Ronin" was cancelled, but it remains unclear whether Marantz's showcased designs were related to that project.
Marantz has worked on multiple superhero films for both Marvel and DC, and he also showcases designs he created that were not used in the final productions. The Krang design revealed this time differs from the version in Teenage Mutant Ninja Turtles: Out of the Shadows: in that live-action film, Krang was portrayed as a brain-like character housed within the chest of a large robot.
One of the concept designs closely follows the approach of early Krang toys, using a relatively simple mechanical body. The other draws inspiration from the Utroms in the Mirage comics: these brain-like beings pilot humanoid robots to disguise themselves as humans. Marantz added arms to the robot based on this concept.
In the comics, the Utroms naturally have this appearance; the TV animated series gave Krang a different origin. What Marantz showcased this time are unused concept designs, and the film project they belonged to remains unspecified.