The Megaladon [ Ancient Megalodon`s Offspring ]  

You can't miss it when it pops.. its likely a small mountain that moves in the sea.
btw, you don't really need to beat it if you are not after the hunter achievement.
Both the belt and the aug dropped from it can be obtained from group task, Rebel Assault and The Herald of Oratory.
Those tasks are pretty easy to compare to spawn and kill it actually.