This is something that I see come up quite often in writing circles and I have a problem with it each and every time I see it. Recently, I saw someone ask how you would fit leprechauns into a science fiction story. Well, you can’t. If we look at the characteristics of what defines a leprechaun, we have magical little men that have a pot of gold at the end of the rainbow. None of that is science fiction, it’s fantasy. You might be able to fit it into a science fantasy story, but that’s not what the original question asked. So can you just redefine what you consider a leprechaun? Get rid of the magic, get rid of the pot of gold, or find some way to make it all technological, not magical. Does that solve the problem? Well no, because you’re no longer talking about leprechauns, are you?
That’s really the thing, these entities are defined by their fantasy characteristics, once you take those characteristics away, you’re no longer really talking about those entities but something else. The same goes for unicorns and dragons and wizards and the like. Take away the magic and you take away the fundamental elements that make up these creatures. You can certainly have them in your science fiction story, but they’re only superficially related to the critters you started out with.
Of course, there’s nothing wrong with taking a concept and making it your own, every writer does it, they borrow ideas from pre-existing sources, but just like Stephanie Meyers did with Twilight, once you make your vampires sparkle in the sunlight, you’ve lost some of what makes a vampire a vampire. Now you have something else.
I just wish people would see it that way. I usually get a bit of argument from people who desperately want dragons in their science fiction, rather than large winged lizards. I usually get people complaining when I point out that the fanciful magical creatures they’re using aren’t really suited for sci-fi, no matter how hard you try to rationalize the magic. Some things work with science fiction, some things do not and when you try to make the things that do not into things that do, you’re just changing from one critter to another.