Megapiranha
As one of the deadliest river monsters, these extinct animals originated in Argentina. They were estimated to weigh 10 kilograms, and 71 cm in length. This genus of piranha did not hunt in packs, like the Black Piranha, and may have been solitary hunters. when the Andes Mountains rose, they separated the two basins, which scientists think caused mega-piranha to go extinct. Their habitat shrunk when their watery lairs turned into little pockets. There were no longer prey resources available to sustain its body size. Either way, we’re lucky we won’t run into this piranha while swimming (River Monsters).