What is the difference between a horse and a unicorn?
If you said that one has a horn and the other did not, you would be wrong. The real difference is that one exists and the other does not.
In some ways, this example can be used as an argument for nominalism, the theory that objects which “embody” universals only have their names and concepts in common.
This is a rather strange way to phrase it. For to say this is what is common is not to talk about how the universal came about in the first place.
The more likely route to the universal is that two objects were perceived to have the same quality and so a term was given to this quality. That is, the creativity is first to see the similarity and differences then to make the distinction by naming this difference.
So in the case off the unicorn, it is not a horse with a horn, but a horse-like creature with a horn. To say the term “picks out” a unicorn is to make it “real”. But to say we are labeling an imaginary horse-like creature a horn “unicorn” is to properly understand how terms come about.