The Dunkleosteus is an extinct genus of arthrodire placoderm fish that thrived during the late Devonian period, 350 MYA, although it is possible, though highly unlikely, that this monster exists now.
The beast was said to apply an enormous force, much greater than the Megalodon, but only oversized specimens could do that, while most did only 7000 N per square centimetre per bite.
Unusually for a Devonian fish, the Dunkleosteus was, due to it's heavily armoured nature, was a slow but powerful predator who could slice through their own bone shield with one bite, though they usually stuck to eating invertebrates, which they crushed using their jaws.
The Dunkleosteus had no teeth, instead they had 2 super sharp bony plates that could crush and slice anything, as evidenced by the horrendous but marks on some creatures they prayed on.
In 2003, the Kaikō submarine disappeared during a dive into the Mariana Trench, when it took a picture of a 60 m long placoderm swimming nearby, and soon the robot was crushed and the winch cleanly bit in half.
The footage that Kaikō took was analysed, and soon deleted to avoid panic, so it is now impossible to figure out what it actually was, but the Dunkleosteus fits into this category nicely.