Specifically, the monster in http://dmfa.katbox.net/cubi-mind-abilities/cubi-mind-abilities-4/ is what I'm asking about. Is that anything that's ever been elaborated on?
Eh. He's a particular type of Mythos if I remember correctly. They're pretty much the best of the best when it comes to mental projection and psychic travel.
Mythos, huh? So I guess that means they were just mudding around in the same dream, then? I took it for a monster that exists purely in the dream world, instead of one of the normal races. Like something which lives in nightmares and eats dreamers. That would probably be double-bad for some invasive cubi. Still, my question's answered.