What mythical creature should exist and why?

I think vampires but, only if another equal opponent exists to balance the food chain.
There are several things that one can do that a regular lifespan would never allow them to do. Being injured so easily prevents one from doing many things as well.
If you ever got bored of your life, if you don't get killed first, there are methods to end your life. Go pick a fight with the government or something.