Megalodon
This is the real monster, what lived approximately 2.3 to 2.6 Million years ago in the ocean.
it is a shark.
It had the sharpest and biggest teeth with a giant 59' body. it could sink a ship like Titanic damn easily.
Their large jaws could exert a bite force of up to 110,000 to 180,000 newtons (24,000 - 41,000 lbf). Their teeth were thick and robust, built for grabbing prey and breaking bone.
The animal faced competition from whale-eating cetaceans. it is thought that oceanic cooling associated with the onset of the ice ages, coupled with the lowering of sea levels and resulting loss of suitable nursery areas, may have also contributed to its decline
Eventually the Great white shark was megalodon's food. when that monster used to live in he ocean, no monster was there but that.
even you wouldn't believe, That monster could eat blue whale like a chips
References
i have taken some information from here