Why is it called "fish"? In fact, there is no definite answer. There is a saying that ancient people thought that the fish's body shape and swimming style were very beautiful and flexible, and they could be regarded as spirits in the water, so they named it "fish". Another way of saying it is that fish is one of the main foods of ancient people, so it is called "fish".
Generally speaking, the word "fish" has been widely accepted and used as a general name to describe vertebrates in water.