I showed my ignorance about these “dragons” and my sister Noelle gave the following information. It was too interesting to keep to myself:
Noelle’s answer: Monitor Lizards, related to Komodo Dragons.

Rest is with help from Wiki:

The Water Monitor, (in same genus as Komodo Dragons), capable of growing to 3.21 metres (10.5 ft) in length, with the average size of most adults at 1.5 metres (4 ft 11 in) long.

In Thailand, the word water monitor or actually local word ‘เหี้ย’ (hia) is used as an insulting word for bad and evil things including a bad person. Its name is also considered a word bringing a bad luck, so some people prefer to call them ‘ตัวเงินตัวทอง’ which means ‘silver and gold’ in Thai to avoid the jinx.

The origin of this offensive meaning can be traced back to a time when more people lived in rural areas in close proximity to monitor lizards. Traditionally, Thai villagers lived in 2-story houses, the top floor was for living while the ground floor was designed to be a space for domestic animals such as pigs, chickens, and dogs. Water monitors would enter the ground floor and eat or maim the domestic animals, also hence the other name ‘ตัวกินไก่’ (Tua kin kai – chicken eater).

Noelle again: the creepiest thing is when you look down into the sewer drains, and see their yellow eyes peering up at you. I think they’d make a great movie.