Chapter 38: Piscciss Volann
From the start, they should be impossible. Since their planet's gravity is too weak to hold itself together, they shouldn't even exist. However, since the planet was created thanks to a gravity enhancing device placed by Professor Paradox, most likely inside a spherical hollow asteroid, the planet formed. These guys also don't have need for legs. It would make a certain amount of sense if they had land too, but they don't. Fish don't even mate, so there is no reason. They also don't seem to need gills, as they have lungs that could do it for them. Unless these lungs are able to breathe air, the gills seem useless. However, gills allow breathing underwater while swimming at fast speeds, something lungs don't. They seem to have an extending skin, shaped like a loin cloth, which slides over the legs to make an aqua dynamic lower body. I think that they don't just randomly grow fins, but that the feet have an extra joint that allows them to turn into flukes. This would give the tail only the flexibility of the legs. The need for legs is fictional, since there is no land to walk on, no nothing. The only reason for legs is mobility on land. They have arms to grab stuff which evolved from fins. The rest of their abilities are quite natural.