I realize the whole fish/dragon thing in Chinese mythology, but Oda has been consistent about the species of the models matching that of their base form. Reptiles and Fish are completely different species.
Even the ones that look wrong are right. For example, 9-tail fox belonging to Inu, is not "Inu" as in dog, but "Canine"/"Canid" which foxes belong to.
https://ja.wikipedia.org/wiki/イヌ科
As for Dragon/Dinosaurs, they're both reptiles/lizards. Wikia explains this
- Ryū (竜?) is Japanese for "dragon" and can be found in the Japanese term for "dinosaur", kyōryū (恐竜?).
Fishes on the other hand just have nothing to do with dragons full stop. Just because X turns into Y in mythology, doesn't mean you can have X X no Mi: Model Y. That makes no sense.