Kishi originally intended the Akatsuki to be a group of monsters, and you can even see that in some of the designs as we know them today: Deidara has mouths on his hands and chest, Kakuzu is comprised of thread, Sasori is a human puppet, Pein is actually one person controlling six bodies, Zetsu is a plant-like incarnation of both Hashirama and Madara, and Orochimaru, while only being a defected member, is not really human, but is technically a giant snake in a human's body. Honestly, from most to least normal when compared to an obviously-human fodder, we have: Itachi, Konan, Nagato, Tobi (Obito), Deidara, Hidan, Six Paths of Pein, Sasori, Kakuzu, Kisame, Orochimaru, Zetsu (this is all just how close to a normal human I consider them). Because Kisame was one of the earliest-introduced Akatsuki members, he was made to fit the intended "monster" image.