If someone gives me a new creature, there are things that I can do to ascertain whether it is a fish. The only question is how quickly I could do this.

I'm guessing you'd quickly say "yes" for Panderichthys and "no" for Acanthostega... but what about Tiktaalik? Or if that's too easy to answer (which answer?), pick any clear amphibian and start looking at its ancestors. Is there a clear line where "this is not a fish, but its mother is"?

We think of ring species as rare populations with interesting spatial distributions, but thanks to common descent every living thing is part of one big multi-ring species with a very interesting space-time distribution. It's hard to categorize living things, in part because the obvious ideas for equivalence relations turned out to not be inherently transitive.