As mentioned in the character description section for Cooper, according to "Cooper's Rule", if a human is awake when exposed to the mutagens, they will turn into "nightmare fuel", i.e. - "a monster". So far, monsters have ranged from what appear to be pools of goo, to things like a big octopus and really large hungry shapes. To date, only one type of monster has been identified by name.
Or pixel monsters are microscopic creatures that act like semi-intelligent monitor pixels. When millions of them work together, they are capable of displaying any image they want as either an illusion, a mirage or a hallucination. Images can be of other monsters, vehicles, people or of simple, vast open space. As shown in the Gustine quest arc, dittos live in "nests", and are susceptible to raygun blasts. As shown in the flashback sequence with Sparkplug Sanders, dittos can read people's memories and play them back in full 3-D. As shown in the Countdown arc, dittos can be harnessed and trained to perform useful tasks for humans and/or mutants. So far, they have not demonstrated the ability to produce sound. Topsider Personnel Transporters identify them as "vermin" and have protection systems in place for killing them.