Are there any poisonous / dangerous animals in Thailand?

Mostly, poisonous animals are found in forests.

While participating in an adventurous trip in or near the forest you should be aware of dangerous animals, such as snakes and centipedes. While there certainly are deadly snakes in Thailand it is extremely uncommon for visitors to see one, let alone be attacked and killed by one. Furthermore, there are no man-eating sharks endemic to Thai waters and one’s risk of being killed by a wild tiger is far lower than a road accident. Dangerous wild animals are not a serious concern for travelers to Thailand. The following are the most dangerous animals you may come across. Thailand has poisonous snakes, scorpions, centipedes and jellyfish. If you see a centipede, do not try to hold it or touch it, they have an extremely painful sting and if you are stung by one, you will be off your feet for days.

Scorpions like to hide in clothing that's been left on the floor, in shoes, under logs etc. Snakes can turn up anywhere, even in the cities. If you are bitten, call for help immediately but try not to panic as snake bites are easily survivable and treatments are available everywhere. You should use caution when bathing in the sea. Swimmers have received fatal stings from jellyfish. Certain varieties are very dangerous and are found in coastal waters all around Thailand. Generally, jellyfish stings are just painful and don't pose a threat to life. but you should be aware of the dangers.