Any monster appearing in a dream represents something that we have made larger than life. We have personalized it so that whatever is worrying us appears as a creature.
It usually indicates our negative relationship with ourselves and fear of our own emotions and drives.
If we vanquish the monster, it is said that we must take care not to kill it, since we may be killing off part of our personality. Equally it should not be allowed to overcome us; self-survival kicks in and we will normally wake up before it does.