Ngueak

Ngueak (derived from the Sanskrit Nāga(नाग) "cobra" or "snake") are mythical animals resembling mermaids once widely believed by the Thai people to inhabit rivers. They range in length from 30 to 50 feet (9 to 15 m) and have a poisonous bite. They have the power to suck all the blood out of a human or animal when they drag their victim to their underwater caverns. If a body disappeared in the river and does not float, the Thai used to say that a ngueak has got it.

The culturally similar Lao people worshipped similar snake gods known as ngeuk. They believed that ngeuk had the power to overturn boats. Fa Ngum, founder of the Lan Xang state, considered to be the first Lao state, performed sacrifices to the phii (traditional spirits) of the kingdom and the ngeuk of the Mekong. Later, a scheming queen, known only as Mahathevi ("great queen") is said to have killed off a succession of youthful kings before ruling herself. She was later overthrown by the nobility and sacrificed to the nguek by being chained to a rock and drowned in the Mekong. Most Lao peasants still believe that ngeuk exist.

These creatures are also believed to be responsible for creating naga fireballs once a year in the Mekong River to celebrate the end of Buddhist rains retreat in October.

References

"The pearl of Asia: Reminiscences of the court of a supreme monarch; or, Five years in Siam", Jacob T. Child, 1892 [1]
"The Lonely Planet: Laos", introduction by Professor Martin Stuart-Fox