After Zhou Wen figured out the situation in the Seven Dragons Sea, he realized that they weren't proper dragon monarchs.
Miya was already considered the most normal dragon in the Seven Dragons Sea. According to her, the dragon monarchs in the other six dimensional zones didn't have true dragon forms.
Some looked like crocodiles, some looked like lobsters, and one looked like a whale. However, because all of them had some dragon blood in them, they called themselves dragon monarchs.
That Seven Seas Dragon King shouldn't be a purebred dragon king. In Zhou Wen's impression, a purebred dragon king had to be like the white dragon in Dragon's Well.
If it were any other time, Zhou Wen wouldn't have gone to unknown dimensional zones to kill the dimensional creatures. However, with Miya's intelligence, things were different.