Detail from a map in the atlas Theatrum orbs terrarium, Abraham Ortelius, 1583. Old fears of the unknown are reflected even is this revolutionary work of science. 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.