I think most of them are real in the DCU. Most every religious mythical creatures are for sure real. I think there should be stories where a superhero detective like Bruce, Ralph, or Bobo are trying to investigate the existence of a creature like Mothman. At the end of the issue they can’t find any proof and conclude he doesn’t exist. As the detective walks away into the horizon, the reader sees Mothman staring back at them from the foreground.