Beast. Your animal nature is becoming too strong, too frightening, meaning that you are afraid of your own strength and drives. It could also indicate a person (Father or Mother) who appears overly powerful.
Talk to the frightening monster in your dream, have it out with him and observe him.
Mythical creatures, like dragons and ocean monsters, point to moral conflicts that may cause personality disturbances. Seek therapeutic help when these images appear often and include a great amount of fear.