What is a person or thing that has an imaginary or unverifiable existence?

An example of a person or thing with an imaginary or unverifiable existence is a unicorn. These mythical creatures, often depicted as horse-like beings with a single horn, are widely recognized in folklore and popular culture but have never been proven to exist in reality. Similarly, fictional characters, like Sherlock Holmes, exist within the context of their stories but do not have a basis in real life. Such entities can serve various purposes, including entertainment, moral lessons, or cultural symbolism.