11. Jaekelopterus
The Jaekelopterus was an incredibly large sea scorpion that lived just under 400 million years ago. They were among the top apex predators of their time and even ate others of their own species. They were bigger than modern-day humans too, measuring out at 8 feet in length.
The Jaekelopterus were accompanied by massive, spiked claws that were fast and capable of catching anything they wanted. Jaekelopterus are arthropods along with insects, crabs, etc. Thus far, it is the largest ever discovered from this species type. Naturally, you can understand why they are among the most horrifying sea monsters in history.