Water monitor lizard. These lizards normally grow to 1.5 - 2 meters, weighing over 50 kg.
Here's a few images taken in Phetchaburi along with a short clip.
These two images are of a Eastern Garden Lizard (Calotes versicolor) also known as Garden Fence Lizard, Oriental Garden Lizard, Indian Garden Lizard, Indian changeable lizard and Variable Agama. Kaeng Krachan National Park.