The "Koopa Creature" featured in the climax of Super Mario Bros. is definitely one of the film's most impressive accomplishments. This is even more impressive considering the amount of work it took to provide the four-stage de-evolved creature only for it to be featured for mere seconds toward the end of the film. While earlier drafts of the script featured a slower progression of Koopa into this creature or an extended climax on the Brooklyn Bridge, the end result had to be tweaked due to time and budget restrictions. The writers came up with the "bucket scene" and the later stages of the creature were designed with this in mind.
For reference, here are descriptions of the four "stages" of the creature, directly from Rob Burman, who was contracted to create the creature:
Stage I: A large, lizard-like smile prosthetic on Dennis Hopper.
Stage II: A prosthetic overhead with a plastic mouth insert and radio-controlled lip-pulls to enhance the smile and full contacts. This was put on a different performer that was more the right body shape for Koopa.
Stage III: A Dinosaur-like radio controlled puppet with an additional operator underneath for the larger head movements (Me!).
Stage IV: A larger radio-controlled puppet head looking more T-Rex-like. After that, it was all CG animation. 13 weeks of work for 13 seconds of screen time.
So, feast your eyes on these exclusive photos of each of the stages and appreciate the amount of work that went into each one. Look closely and you'll see some alternate paint schemes that were attempted before the final version as seen in the film. For more information on the entire process of getting this creature on the screen, be sure to check out our interview with Rob Burman.