Despite the name and appearance, that is not a reptile, but actually a whale (and not even the most fearsome on the list!) Basilosaurs were predatory ancestors of modern whales, and could be 50 to 85 feet long! It is described as being the closest a whale has ever come to being a snake because of how long and sinuous it was. Imagine swimming in the ocean with an 80+ foot long alligator-snake-whale. Now imagine being afraid to even take a bath ever again.

Physical evidence suggests that basilosaurus did not have the cognitive ability of modernwhales, nor the ability to echolocate, and could only navigate in 2 dimensions (so no deep diving or breaching). So at least this monster whale was dumber than a bag of prehistoric hammers and could not chase you if you dove or scrambled out on dry land, probably forever.