(Movie adaption) Everything is a comatose dreamLaura, the girl who threw herself from a building at the start of the movie, actually survived the fall but fell (no pun intended) into a coma; the entire plot is something dreamed up by her subconscious as her brother visits her, bringing red roses and reading the Sleeping Beauty to her. It all makes sense now!
(Movie adaption) The monsters were taken from Timothy's dreamsIn the movie adaptation, the AI Alice, whose designated purpose is monitoring & controlling the dreams of everyone in the facility is ultimately responsible for creating the hellish apparently Post Apocalyptic world they waken to. Timothy instantly recognizes the hellish bat-creatures and the reptilian monsters as creatures from his games, and his knowledge of them helps them survive — particularly the first time one appears, where he explains it hunts by sound rather than sight. It's strongly implied that Timothy is a total game-nerd, so it's very easy to believe he would have been dreaming about them in the cryo-tube. Alice's directives being what they are, it makes sense that she would scan Timothy's brain while hypnotising him to become the guide and use the dreamed-up monsters to further defend Shizuku.
"Medusa" is a wad of Tyranid; heavily corrupted by Chaos.A combination of Body Horror and The Heartless; it fell through the cracks of the Warp "long ago in the future" (time has no meaning in the Warp) and fell to modern-day Earth as a comet. Dreams and Nightmares become infectious reality, although bound by biological limits.