26. Jasconius
The Jasconius is well known among the horrifying sea monsters sector due to its popularity in Roman mythology. There are even some who believe it is mentioned in the Holy Bible revolving around the story of Jonah. The story of Jasconius is pretty compelling. Original sailors routinely came across big creatures, often embellishing their stories when told later.
The Jasconius is mentioned by Ancient Greeks who claimed it was able to crack ships in half. Many Greeks assumed the same creature was involved in the connected attacks. The Jasconius likely did exist, but it was nothing more than numerous different large whales they once sailed near. It could be what swallowed up Jonah. However, it was unlikely to be the exact same whale.