Arapaima are South American. They are in Thailand because Thai fishery owners stocked them, and they have thrived in a climate close enough to home that some now reach genuinely huge sizes. It is artificial and nobody pretends otherwise, and it is still one of the few places on earth you can realistically catch one. Their native range and biology are set out by the Mekong River Commission, which tracks introduced species across the basin.
They are air breathers, which changes the whole experience. Every fifteen minutes or so a fish rolls at the surface with a sound like a bucket being emptied, and you can watch a lake and know roughly how many are in front of you before you cast.
That surfacing habit is also how they are targeted. Rather than fishing blind, you watch, mark where a fish is working, and put a bait in its path. It is closer to stalking than to sitting behind rods, and it suits people who cannot bear waiting.
The fight is violent and short by comparison with a big carp. Long runs, several complete jumps clear of the water, and a lot of broken tackle. The scales are hard enough to have been used as tools, and a thrashing fish at the net is genuinely dangerous to be close to.
Most of the lakes holding them are outside the city, an hour or two out, and are worth the drive for the setting alone.