Grendel

The creature from Larry Niven, Steven Barnes and Jerry Pournelle's The Legacy of Heorot .

Grendels are crocodilian in appearance and behavior, with jaws that can crush steel. Their bones are significantly stronger than those of humans, as they are not based on calcium. They have a sense of smell better than a dog's. Studying its brain shows it is not fully sapient, but that it is not far off, and is at least as smart as a gorilla. Its claws are not just weapons, but exert enough traction for the creature to sprint up rocky cliffs. Though it is not a true amphibian – it cannot breathe water – it does possess an integral snorkel enabling it to move undetected beneath several feet of water. Its cardiovascular system and musculature give it strength and stamina far beyond that of humans, and that is without its primary evolutionary advantage: A super-oxygenated blood supplement.