I really like this artist's depiction of how Seb's lower end works. Of course, I also love the typical cetacean/slit-related interpretations the vast majority go with, but this way aligns more with how I imagined his mutations actually happened. I always imagined his tail essentially being the result of his legs fusing together, elongating, and growing fins. So the huge tail would start from the thighs down, and he would still have humanoid junk and vestigial hips and thighs, like this. I just can't imagine any other way human legs turn into a giant fish tail, other than just surgical replacement.