I see. Is there a specific ability required to see them?
[As for the whole 'world' thing...]
Marina Nova has the ability to bring in prisoners from various dimensions, worlds, and periods of time. It is difficult to explain beyond the idea that different universes do exist. In some cases, it has even brought more than one version of the same person, or people who you may know, but who do not know you because they are not from the same version of the world you know or they may be from a point in time that was before the two of you met.
[He wasn't sure if that would be easy to understand, as it was a really abstract topic, and many had difficulty accepting the idea, even those with artificial or higher than usual intelligence.]
As such, where I am from, my "world", does not having monsters in the real sense of the word. It is a fictional construct often used for scary stories.