The genesis of this design came from a couple of different sources.
I wanted to create a bad guy in he same vein as "Jaws" from the James Bond films- someone implacable and unstoppable. Something that is just a pure unthinking, unrelenting killing machine. Large yet silent, brutal yet deadly efficient. For that, I felt that a nice homage would to make a shark themed villain, in honor of "Jaws" name. I also wanted to create a "Sentai" inspired type of villain- a bad guy that was more visually compelling than made a lot of sense in terms of aesthetics. With those two concepts in mind, I went about creating Mako here.
I knew there were some things I definitely wanted to include. I wanted the character to have a singular eye slit, to aid in his inhuman nature. Plus a rictus, shark like grin. I also knew that I wanted him to be cybernetic, with the two parts incorporated into each other. I also wanted to give him a larger, more solid upper body, to aid in both his physical presence as well as his alien nature. It took some refining, but I think it came out well. The thing that worried me the most was the fin on his back, but I think I managed to angle the body well enough that it it showcased it nicely.
This was the one I was most worried about, but I'm pretty happy with how it came out, and it managed to look more "shark-like" to me than I originally thought.