Ooo, I know this one! Fish! Also, trees.
People love to annoy people with the fun fact that palms aren't true trees, but are actually grasses (hell, grass probably is one too), as if tree doesn't just mean "Fairly rigid plant that will usually grow taller than humans." It's useful to have words to describe categories of organisms based on how they appear to us and their relationship to us (say, "gut flora"), even if they don't describe coherent clades.
Sometimes it's important to know that a screwdriver was developed separately from a hammer and really has a very different purpose and theory behind it, and sometimes it's important to know that they both go in the toolbox because they're both things you hold and use on other things to attach things together.
My hot take is that just because a term develops a more specific, scientific definition doesn't mean that the existing colloquial definition is wrong, it's just a different use of the word. It's ok for you to disagree with that, but it's strange that you're so very dismissive of the idea.