Oceanic Lung

The cold oceans off the sea of distant Japan teem with fish and squid. The undersea bounty feeds a true leviathan of the ocean, the Oceanic Lung. Related to the Japanese subgroup of Lungs, the Oceanic Lung can grow as large as 30 feet long, and uniquely amongst dragons, they give birth to live young (an adaptation to their aqautic lifestyle) and they protect their calves as best they can for up to three months. The horns are vestigial, as are the back legs, though the front flippers are useful for steering. Their scales are small and smooth, an adaptation to allow for a streamlined body.
The smaller specimans have been observed resting in the shallows, sunning themselves, but adults almost never enter shallow water. Adults can and will tackle anything large enough to fit in their mouths, and stories from whalers tell of Ocean Lungs attacking whale calves. The Lungs are also fiercely territorial. The Oceanic Lung is worshiped by the people of Japan as a deity in the shinto religion, and if they catch a young one in their nets they hasten to let it go for fear of offending the god.

-Trismagistus, 19th century