The Aspidochelone or otherwise known as the Asp turtle or the shield turtle is a gigantic monster. It uses deception, it lives in the oceans, its shell when sleeping is out of the water and can be easily mistaken for that of a small island.
There are a few reports of these beasts the first was by Alexander the great in India. A group of his most trusted men went out to an island that was said by locals to have a hoard of a dead king. The island sank within an hour of the men landing upon it. A few days later Alexander saw the huge tusked beast that they had thought was land.
Sinbad also encountered one on his adventures. Sinbad and his crew had lit a cooking fires on a small island, when suddenly the island shook the monster awoke and dived under the surface Sinbad and some of his men didn't make it back to the boats and were dragged under with the pull of the beast diving. Sinbad was later washed ashore a real island.
There is also another report of one of these creatures from a group of Irish monks on a sea voyage to America they wrote that they had encountered a creature, which looked like an island but vanished under the waves of the sea.