If you dream of a monster, and you have not been taking prescription drugs or recreational drugs, then it means you have experienced a monster/ or heard of myths about extraordinary creatures like monsters. Dreams continue themes in waking life and when you have some contact with stimuli related to monsters, this theme continues into your dream. People who consume medication or drugs often have dreams which are colored by chemicals. This type of dreams do not qualify to be analyzed as they are fake impressions in the brain, which were caused by chemicals.
The dream symbol of a Yeti or monster, means you have been influenced by this symbol in your waking life. Perhaps you should not think too much about monsters and fables for these do not exist in real life.