If you take the tree of life showing the evolutionary branch, you won’t be able to pick off a branch that includes all of the things you would think of as “fish” but excludes all of the things you think of as “not fish”.

The reason for that is that all land animals with a four limb body plan, including reptiles, mammals, and birds dinosaurs evolved from a family of fish called tetrapods. But there are still tetrapods in the ocean that you would think of as “fish”, and I don’t mean whales.

Hank Green has a much more entertaining and complete discussion at https://youtu.be/-C3lR3pczjo