This strange look could be related to the clan kisame belonged to, about which no one knows though. This could be due to the creator wanting to add some difference . Kisame is a water style user so maybe the creator just wanted to emphasize it. Theres no official reason to support this look of Kisame.
Kisame has the ability to fuse with Samehada: Samehada performs the same function as a tailed beast, constantly flooding Kisame’s chakra network with stored chakra. Moreover, Samehada is a sentient, separate being from Kisame: Which makes Samehada a viable tailed-beast substitute.
Throughout the Naruto series, some of us might suspect that the only shark-man in the series is Kisame only, maybe at some point, he underwent a weird mutation that changed his appearance. But in the Boruto anime series, it showed that he’s not alone.