The awesome thing about mythology is that many cultures who had nothing to do with each other in the distant past still managed to come up with similar mythological creatures. Like many different cultures have some version of a vampire or werewolf or shapeshifter or ghost. The details are different but the basic gist is the same. I feel like you can totally make up your own creature and if it's similar to ones that are already made up, that's okay. You have to make sure you're actually creating a new creature with traits similar to established ones, not just putting a new spin on an old myth (like sparkling vampires). There's nothing wrong with putting a new spin on established creatures, you should just know which it is you're doing.
Did you now there was a pre-Dracula vampire that needed the moon to survive? You can do something interesting with that. Research leads to endless possibilities.
A consistent history is hard. I explain my mythology to my husband, and he's like "But what about?..." there are all these things that need to line up. It is fun though.I remember reading somewhere years ago that some culture believed vampires had blue fur. I want to say it was ancient Egypt, but I can't remember now. That would certainly be a new direction in which to go.
I created my own species for my WIP, which are similar to lycanthropes in that they have animal features, but that's pretty much where it ends. I find actually creating creatures or twisting existing creatures isn't too hard for me, it was the developing the history to back it up that was difficult.
You'll likely riff off an established myth but you can twist things and make them different in so many different ways - merging ideas (conflating vampires with succubi or sirens for example), changing them (making vampires protoplasmic), reversing them (making the vampire the hero instead of the villain) and other things can take something established and make it new and interesting.
So, maybe start with the creature you think is most like what you need and consider how to repsray it and change the serial numbers enough to make it yours.
A consistent history is hard. I explain my mythology to my husband, and he's like "But what about?..." there are all these things that need to line up. It is fun though.
It's very, very difficult to come up with something that shares no features with any established mythology, just by virtue of the fact that we humans have been doing this for tens of thousands of years. But it's certainly possibly to create something that isn't just an elf or a werewolf with the serial numbers scraped off.