"Otherkin" is the name we have for people who identify with a fantastic or non-human identity. In this category, we have people who base their lives upon being Klingons, elves, fairies, werwolves, vampires, or "furries". Now, sometimes it's just for fun, and sometimes they take it seriously enough that they get offended if their role-playing alter ego is not treated respectfully.
But we have to admit, if you're a pro surfer, mermaid is the otherkin for you!