There are two species of monitor lizard in Bukit Lawang. The most commonly seen is the water monitor, but this one here is the rough-necked monitor. It's called rough neck because it has scales on its neck.
The rough-necked monitor grows to around 1 metre, so it's a little smaller than the water monitor. As you can see here, they are expert tree climbers, and they spend much of their time in the trees. They eat insects and small animals. Like the water monitor they can also swim.
This is a South East Asian water monitor (biawak air) in Bukit Lawang. Water monitors live on the land, but can also swim very well. They are carnivores, and eat eggs, birds, fish, frogs, and other similar sized foods. They grow typically up to two metres long, and male and female water monitors look very similar.
Water monitors are not Komodo dragons, and they are not considered aggressive or dangerous to humans, but if you handle them they might bite you.
They are also not iguanas, which are not found in Indonesia at all.