Life is created, presumably starting from one species. Then there's an explosion of life. Now all kinds of life. Some have fins. Some have feathers. Some have feet.

Some develop into something else, as different to the ones with fins as humans are. But that design fails. Better to have fins. Now they have fins.

Humans come along and see it and go hey, that's a fish.

But this 'fish' maybe used to be a lizard and then became a fish. So it went:

Step 1: ancestor

Step 2: something else

Step 3: something else

Step 4: lizard?

Step 5: fish?

Meanwhile, 'fish' #2 went:

Step 1: ancestor

Step 2: fish?

So you have one fish that came from a lizard and one fish that came from something else entirely. As a result, you have one fish that has a shared common ancestor with humans say maybe one billion years ago and another fish that has a shared common ancestor with humans three billion years ago. That means fish #1 and humans are related by one billion years while fish #1 and fish #2 are related by three billion years.

Conclusion: some fish are more closely related to humans than to other fish, and the category of fish is meaningless.

This is also true of crabs and trees. Mother nature proves to us over and over again that crabs, fish, and trees are the most superior earthly life forms.