How did catfish get its name?

Because language is arbitrary and most Words do not have any kind of resemblance to real-world objects. Words are symbols, in the same way the letter W represents a certain sound, but there's no reason why it couldn't represent the sound represented by the letter M if everyone agreed that it did.

If you changed your name tomorrow you would still be the same human being, just with a different set of sounds as your name.

The Word "fish" comes from Old English, and before that from Germanic.