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9. Cetotherium
Whales and sharks aren't known to be the best of friends, as you can imagine with large sea creatures that control certain territory in the water. Thus one side tries to defeat the other, or they constantly fight over territory (like with Orcas and Great White Sharks in modern times).
How does this pertain to the Megalodon era? The Megalodon was massive, bigger than any shark today, so surely there wasn't a whale that could take it out way back then, right? Wrong, as the Megalodon had an enemy in an extinct whale species called the Cetotherium.

8. Zygophyseter Varolai
The Zygophyseter Varolai has a more simplistic nickname if you're having trouble pronouncing it...the "Killer Sperm Whale", and one look at its teeth will show you why that name is rather accurate. Just as fascinating is that this impressive beast wasn't even known about until 2016! That was the year when a fossil of this whale was found in Italy, and it certainly astonished a lot of people, especially scientists and biologists. As for why it's called a "Killer Sperm Whale", it's because it's related to, and somewhat resembles, both Orcas (Killer Whales) and Sperm Whales.

7. Liopleurodon
The Liopleurodon was a big creature in the water, but there is actually some debate between scientists as to whether this massive creature was only 20 feet (6m), or actually 50 feet (15m). The general consensus is about 21 feet at a maximum, but other scientists have looked at it and believed it to be much bigger. Regardless of size, this reptile of the oceans was known because of its rather terrifying teeth. Its mouth was full of long, sharp, and pointy teeth that it would use to eat prey quite swiftly, making it a scary beast by any stretch.

6. Basilosaurus
Despite its appearance, the Basilosaurus is actually very closely related to a whale, and it was a very aggressive one at that. Plus, just like whales, the Basilosaurus was huge, with its average size being about 5055 feet (15m) in length, based on scientific estimates. The Basilosaurus also had razor sharp teeth. Its body was very fluid, so in some ways, when it moved, it was more like a snake. A gigantic water snake, big enough to swallow you whole! Admit it, if you saw something like that swimming towards you in the water, you would probably just give up and hope that it ate you quickly! This kind of swift wriggling movement isn’t you would tie to a whalelike species at all.

5. Jaekelopterus Rhenaniae
The Jaekelopterus Rhenaniae has a bit more obvious and startling nickname that might give you pause: sea scorpions. Apparently, this sea scorpion was about eight feet (2.5m) long. That’s a bit alarming when you compare it to the largest scorpion in the world today, which sits at only about 10 inches long (25cm). Doing the math, that means that the Jaekelopterus Rhenaniae is about ten times as big as the biggest scorpion in our world today. What's more, while most modern scorpions have armor in their own right, the Jaekelopterus Rhenaniae had much more, protecting it from those that would try and eat it, especially large fish. Although the Megalodon and this terrifying animal never coexisted, if they did, then the giant shark would have known to steer clear of this bug, or it would have been in for an unpleasant surprise, more than likely.

4. Mauisaurus
The Mauisaurus was a 66 foot long (20m) creature that used to reside in the waters of the ancient world. The creature was actually named after the Maori God "Maui", who played a big part in their mythology. You might know him from the Disney movie, Moana, where he was voiced by The Rock! But the reasoning for the naming of this creature was that it actually had its bones found near the island of New Zealand, where Maori traditional culture is rooted.

3. Rhamphosuchus
In the modern world, crocodiles have been documented to be pretty long, with many getting over 20 feet (6m) in length. But in the prehistoric world, there was a crocodile species that was able to get much longer than that, the Rhamphosuchus.

2. Helicoprion
There are many reasons to talk about the Helicoprion, but what sets this shark apart from others both past and present was that it has a deformed spiral mouth that was actually incredibly terrifying.

1. Livyatan Melvillei
A lot of these ancient creatures would have been weaker or less aggressive than the Megalodon, but this big daddy of giant undersea creatures would not have been among them. This gigantic beast is called the Livyatan Melvillei. It’s a being that has been labeled this as the most aggressive whale species ever to exist.