When you (somehow) breed a Great White Shark with the Leviathan, you end up with a Megalodon, the biggest shark (and fish, other than Leedsichthys). It is 70 ft (20 m) long, with teeth the size of your hand.
One of the greatest things that ever lived. It usually lives in the midnight zone and below, they are very very very rare. When they come to the surface, they terrorize boats and fish alike.