Monitor Lizard

There is no way a Varanus albigularis, a Southern African species, would be found in Malaysia. It is always wise to check official distribution lists against the spotting location before making suggestions.
I would also question the location of Borneo as the Clouded Monitor, which this does appear to be from the position of the nostrils and the small spots on the skin, is not found there but is found in West Malaysia, Java and Sumatra.
The Water Monitor has its nostrils at the tip of the snout and this does not look like the other Monitors found in Borneo.
The Bengal Monitor also looks similar to this but again is not found in Borneo.

Probably some kind of Varanus (http://en.wikipedia.org/wiki/Varanus).

Best,
Lin