Not sure if you're being serious or joking, but for a fun fact: in Japanese lore, a snake that has trained for 1000 years can ascended into a dragon. I am guessing the manga-ka is taking liberation / extending the joke to include any animal. =>
i think theliberty they take in the manga is that they put the process in the meddle (500 years).
you can see in one parte his sister in a lake and her shape is snake like, maybe a water snake, so i think the process in the meddle make you a sharke if you r borne a water snake. i think thats the only liberty in this case.
"Soo, are you a dragon?" - " No, I'm a shark, to be a dragon I need to train a lot more "
Because it's commenly known, that if you train enough you can be a dragon instead of a shark in your NATURAL form XD
Geeeeeeeez I love this manga soooo much <3