That's a dead water monitor.
Supposedly, the critters are the epitome of lizard intelligence. Intelligence, then, is no barrier to wallowing in slime and muck and filth in search of decaying flesh. The Thais focus on the monitor's latter qualities and ignore its maze-navigating talents; refering to another human as a water monitor is a fairly serious insult, something like calling a Frenchman a cow.
The Thais have a number of terms for the water monitor. The insulting term would be "hia." You hear young Thais working "hia" into nearly every sentence, the same way some Americans would use "fuck."
Hia arai? -> What the fuck is that?
Ai hia! -> Oh fuck!
Suay yang hia -> Fucking beautiful
Oddly, the last time I visited the Dusit zoo in Bangkok, the monitor exhibit was labeled "hia", in Thai. My Thai friends are surprised at that.
Up to a few years ago, you'd say "tua ngern tua tong" if you wanted to speak of a water monitor in polite company. It translates to something like "body silver body gold." However, there's an attempt to clean "hia" entirely out of the Thai language and replace it with a new term; "woranut." It'll never work; can you imagine an American government agency declaring that "fuck" will be replaced with a new term?
The genus name for the creature is "Varanus", which is quite similar to "woranut", especially when you consider that the Thai language has no "v" sound. Also, though Thai words may begin with an "s" sound, an "s" letter gets a "t" sound when placed at the end of a word. Unfortunately, "Woranoot" (double o's) has long been a popular Thai name for girls. It simply means "beautiful girl." So now we have thousands of Thai women suddenly finding their name associated with the finest example of disgustingness in this part of the world.*
A number of superstitions relate to the creature. You should walk to the right (or is it left? I forgot) around a dead water monitor. About a decade ago a Thai man made the news when he adopted a monitor based on the belief that his son had reincarnated as one; you'd see nightly videos of the father fondling the creature with exquisite care, heaping affection on it.
The monitors are not-so-distantly related to Komodo dragons, the biggest of lizards. They're ubiquitous in this part of the world. Even when you can't see one, you might hear one rustling in the weeds, or slinking away into a shallow, scummy canal.
*The Thais may be the world's greatest name-changers, though. I'm guessing 50% of Thais change their first name at least once in their lives, making searching for old friends on Facebook a bit more problematic than usual. It's generally the result of a visit to a fortune teller who augurs that one's name is inauspicious, offering a number of choices for replacement. So this problem of suddenly being saddled with a disgusting name can be dealt with relatively easily.