Megalania is a giant Australian lizard. It unfortunately never appeared in the Walking With series but another show similar to Walking With Beasts named Monsters We Met (Land of Lost Monsters is the US title) there it attacked the first humans in Australia. Eventually they kill the man-eating lizard by destroying it's habitat with fire.
It was a huge 7-9 metres long and weighing 2-5 tonnes. The largest true lizard of all time, and the largest terrestrial and non-crocodilian reptile to exist after the Mesozoic Era.