Myths surround us. The idea that there are things in our lives that we can't locate or prove is amazing, especially in this day and age. Whole genres of writing are dedicated to myths along with a couple divisions of science.
How do myths fit inot writing? Research is key here. Every great myth I can think of has an amazing and rich history. Involving that history into your story not only adds depth to it, but it teaches your readers something they may not have known.
Not all stories about myths are true. You can absolutely create a myth in the realm of your story. Make sure the myth you are creating has a great back story with lots of accounts and an air of mystery. Think about the Loch Ness Monster and Bigfoot here. No one has a picture of them that is in focus, yet so many have sworn they have seen these two creatures. All encounters have very similar elements like details about the creatures themselves, or the locations, or the times they are seen.
Go full scale with your myth if you can. Give your creation roots in the real world. Before you know it, your creature may become the next dracula.