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In creating my world, I am trying to write down and create mythical wildlife and monsters. I had been thinking of using sirens as one of them. Not a creature based off of them, but the actual Greek fictional entity. Would it be okay to have specific mythical monsters from ancient mythologies, in a high fantasy world that is not urban? For some notes:
They will be called sirens.
The sirens in my world are the creepy humanoids who pretend to be attractive people, not the flying birds, and try to eat you.
They exist in a region inspired by Greece
The sirens, while can talk through their weird "singing", are really feral in reality and cannot talk.
I don't think this is something you have to worry about doing. It's your world and you can do whatever you want with it. If you're wondering if people will get the wrong idea about what sirens are because of preconceived knowledge, you might benefit from a scene or some kind of exposition explaining them to highlight how they're different.
I know twilight isn't high fantasy, but they "hang a lantern" (call attention to something for the purpose of cueing the audience to notice that something is different than their preconceived notion of how it should work) on the differences between folklore vampires and "real" vampires. Haven't read it, but I believe I've heard that they actually poke fun at the idea that garlic, holy symbols, or sunlight would hurt them as a way of explaining to the audience how the author wanted to pick and choose which vampiric attributes they wanted to keep.
First off, there is certainly no legal issue with it. Unlike monsters from other fantasy novels.
However, remember that your readers are going to expect them to
Be sirens out of Greek legends. If you want to diverge, you may have difficulties managing reader expectations
Be part of a monster ecosystem that is probably heavily Greek. This is less of a problem with monsters so well known as sirens, as opposed to, say, a lamia, but you may want to look at patterns you create (are you, say, having one well-known monster from each culture?) so that the readers will know what is reasonable in this world.
I think it is tricky.
It will depend on how much you describe them when they are first introduced. Make sure to tell or show how they are, so people get a good picture of how they look and act for later in the story. So the reader know what Sirens are in this story.