Most of marine creatures in Himmapan world are combination of a land animal and fish. Perhaps because in ancient time, people associate water with fish. Hence, everything that's able to swim would have fish tail.
Read more on different types of Fish based Creatures: Hema Warin Kunchorn Waree Mucha Naga Mucha Wan Nang Ng-uek Pla Kwai Pla Seau Saringka Mussaya