Megalodon

This is a fairly well known prehistoric monster, but it is just so big and scary that it deserves to be in this list. Megalodon (technically called a Carcharocles megalodon) was a gigantic shark, closely related to todays makos and great whites. It could grow up to 20 meters long and weigh up to 60 tons, being almost six times larger than Tyrannosaurus rex! Obviously, the only thing in the sea big enough to feed Megalodon where whales, and indeed, the giant sharks bite marks have been found in the fossil remains of whales all around the world. Although many people like to imagine encounters between Megalodon and TRex, or dinosaurlike marine reptiles, the truth is Megalodon appeared long after the extinction of such creatures, and it wasnt seen alive by any humans either, although it was still roaming the oceans when our australopithecine relatives took their first steps out of the jungle.