In my initial research into monsters I have focused on three main time periods/cultures.
Greek Mythology – Myths taught by the ancient Greeks, 12th-9th centuries BC to the end of antiquity (c. 600 AD)
Norse Mythology – Mythology of the North Germanic people stemming from Norse paganism and continuing into the Scandinavian folklore of the modern period.
Traditional Chinese Mythology -myths found in the historical geographic area of China, including myths in Chinese and other languages.
Sources (including images):