Megalania

http://australianmuseum.net.au/megalania-prisca

The Megalania or Varanus Priscus was a truly horrifying creature. It grew to be roughly six metres long and had razor sharp recurved teeth. It lived in Australia at the time of the Megafauna and was a predator for many of the animals at the time. It was most likely an ambush predator and is related closely to the Komodo Dragon. Like the Komodo Dragon it has a venom, making it not only the largest venomous lizard, but the largest venomous creature known. This venom like that of the Komodo Dragon leads to an inability of blood clotting and shock in the creature. This means that the Megalania could have more safely killed its prey by biting and letting the prey weaken until unable to defend itself. It is at this point that Megalania came in and devoured its prey. No prey will have been too ferocious for this predator short of Pallimnarchus or Quinkana, and it is still likely it may have taken on those monstrosities.