(Strangely, some mythical creatures, such as vampires and unicorns, seem to be defined in a spiritual way; whereas others, such as mermaids and centaurs, do not. A horse genetically engineered to grow a horn would probably not be thought of as a "real" unicorn; a genenged mermaid probably would be admitted to be a "real" mermaid.)

Dunno if it's because I'm not a native English speaker, but my intuition about the words unicorn and mermaid doesn't agree (whereas it does agree e.g. with Gettier about the precise meaning of knowledge, and most other similar problems about precise meanings of words).