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.
You are being too bold when asserting that this makes no sense.
Ryus are chimeras usually described as having carp scales, catfish whiskers and lobster eyes. They aren't reptiles per se.
Many crustaceans are associated to the Eastern dragon, and we find this connection in popular designs of monsters and even animal names (the lobster is called "dragon shrimp" in Chinese, mantis shrimp is called "blue dragon shrimp" in Japanese, etc.).
The tale of the koi that became a dragon by climbing the waterfall not only is one of the most popular over there, but carps and dragons also share other symbolic properties, such as their common relationship with the sea and immortality.
Wamyo Ruijusho, the oldest Japanese thesaurus and a very important dictionary in the country's history, associates dragons and aquatic animals and calls the latter category "Ryougo" (which includes "ryu", in case it wasn't clear). Their section is commonly translated as "Dragon and Fish".
Jean Chevalier, one of the most renowned academics on symbols, when discussing dragons states how "the aquatic symbolism is obviously primary, and while dragons live in water and make springs flow (...), dragons are mainly related to producing rain and thunder, manifestation of celestial activity". Directly connecting two of Kaido's power main traits, the sky and the ocean.
And I could go on. There's plenty of material about the aquatic nature of dragons and their deep relationship with water animals (fishes and crustaceans, mainly) and it isn't that farfetched that Oda decided to give Kaido the Uo Uo fruit (which we don't know if only includes fishes or all kind of animals that live underwater), especially knowing that he has been consistently connecting this creature to the sea (as I mentioned before: Luffy confuses Momonosuke with an eel, not a lizard nor a snake; Neptune lives in the Ryougo kingdom and his palace has a big dragon statue; the baths in Arabasta have dragon-shaped fountains).
"Ryu" is used in words for dinosaurs, pterosaurs or lizards, but that doesn't make dragons necessarily reptiles or, at least, more related to them than to fishes; this is factually not the case as soon as we check mythology, symbolism and other cultural clues. As I've discussed in other posts, odds are Oda just took the common denominator in the etymologies of some reptiles in order to create the Ryu Ryu family without actually wanting to create a direct reference to the dragon itself.