It’s not a good sign to dream of seeing monsters. If it happens once in a very blue moon, I would chalk it up to old memories from watching TV and movies. If the dreamer sees monsters and other violent dream content in recurring dreams, then other dream analysis has to be tested out. For seeing unreal creatures and violence may signify that the body is unwell. When internal organs are sick, they send signals to the brain, which then tries to assemble images that represent the ill nature of the communication it received from the organs.

Of course, a drug user (or legitimate prescription drug patient) is liable to see monsters and colorful dreams, due to the reaction and interference from drugs. These dreams should be discounted as they are meaningless; being caused by drugs. If someone tells you they have been seeing recurrent nightmares, violence or fighting in dreams, then they probably have internal physiological stress, and/ or a mental disorder.  They need to seek professional help from a qualified therapist.