Rhabdophis chrysargos
This species is found in Thailand, but not within Bangkok itself
Kraeng Krachan NP (Thai National Parks / CC BY-SA) 
Khao Yai NP (Bernard Dupont / CC BY-SA) 
Malaysia (© Wolfgang Wüster) 
Malaysia (Alex Figueroa / CC BY-NC-SA)
English name: Speckle-bellied Keelback (aka: “Specklebelly Keelback”)
Scientific name: Rhabdophis chrysargos
Thai name: งูลายสาบจุดดำขาว (Ngu Lai Saap Jud Damm Khao)
Description: To 98cm long. Brown to olive-brown with narrow black bars that have a light segment in the middle. In some individuals a dorsolateral line passes through the light section of the bars. Front of body sometimes tends towards yellow, yellowish-orange, or greenish. Top of neck has a large dark mark with a characteristic lighter chevron (often white or yellow) coming off of the lower lips and usually continuing until joining at the back of the neck. Head is grey to brown to black with white/yellow lips and underside. Belly is yellow with brown mottling.
Relevant scale counts: 19 midbody scale rows, scales are strongly keeled.
Similar Species: Black-banded Keelback lacks the light chevron and has distinct black streaks coming down from the eye.
Red-necked Keelback lacks the light chevron and has red color in the area of the neck.
Blue-necked Keelback lacks the light chevron and has blue on the sides of the neck.
Range: From eastern Myanmar across all of southeast Asia north through southern China and Hong Kong, south through Malaysia and Indonesia, and into the southernmost Philippines islands.
Habitat: Forest habitats up to 1700m elevation, usually near streams.
Place in the ecosystem: Feeds primarily on frogs as well as lizards, birds, and rodents. Would be eaten by larger snakes, monitors, and birds of prey.
Danger to humans: It is not known to be venomous but other related species have dangerous bites so it should be treated with caution. Information on first aid for rear-fanged venomous snakes can be found here.
Conservation status and threats: Not seen to be under any threats as it is wide-ranging and can tolerate some disturbance in habitat.
Interesting facts: As can be seen in the above pictures, the Speckle-bellied Keelback differs remarkably in coloration, sometimes even within the same population. While the light chevron coming around the neck and dark crossbars with light middle portions are common to all snakes of the species, the base color, neck/front body tint, distinctness of the crossbars, distinctness of the light middle portion of the crossbars, presence of dorsolateral stripes, presence of a black neck portion, head color, lip color, and belly color are all extremely variable.
References:
Ecology Asia: Speckle-bellied Keelback
IUCN Red List: Speckle-bellied Keelback
Thai National Parks: Speckle-bellied keelback
Field Guide to the Reptiles of South-East Asia
Reptiles and Amphibians of Thailand 