6. Liopleurodon
The Liopleurodon was a creature that lived during the Jurassic period, several million years ago. Like others, it was not technically a fish or natural sea-dweller. The Liopleurodon was a giant marine reptile. In the water, there were very few predators that could go up against them and live.
They grew up to 30 feet in length normally but there are fossils that seem to indicate that some grew even bigger than this. In studies of the species, we have been able to find that they had some incredibly big and sharp teeth that could rip through anything they chose to attack. They were very fast for their size and even had a great sense of smell that they used to find prey.