Mosasaur

The massive swimming reptiles called mosasaurs thrash their powerful tails to propel them after prey. Four articulated, webbed paddles let them precisely steer their paths, and their hinged jaws—much like a snake's—allow mosasaurs to swallow larger creatures than their size would indicate.

Platecarpus (Creature 3), Tylosaurus (Creature 8)

Slowly swimming the boundless seas of the Plane of Water, the Void Mosasaur has become legend to that plane’s inhabitants. The creature is ancient, marked with massive gouges and scars, eyes entirely clouded with cataracts, and with a metabolism so slowed that days pass between its heartbeats.