Sea Monsters or Whales? Woodcut from Gesner's great zoological work, Historiae animalium, depicting two sea monsters, 16th century. Sea monsters are sea-dwelling mythical or legendary creatures, often believed to be of immense size. Marine monsters can take many forms, including sea dragons, sea serpents, or multi-armed beasts. They can be slimy or scaly and are often pictured threatening ships or spouting jets of water.