I stopped to take this video after a woman street vendor screamed, it's a 15 inch poisonous centipede which promptly escaped between a gap in the fence. I only managed to catch the last part of it's escape.
After I walked away, another scream, same vendor, she's lifted her mat and found another one hiding underneath.
The reason they're there, is the tree just behind the fence. That type of tree has large spaces between it's roots, and that caterpillar loves to stay there in the moist darkness. I avoid that tree, sometimes these caterpillars fall from it.
But don't let that worry you when you walk under trees in Thailand. Put it completely out of your mind, the riggly crawly poisonous things, forget them totally. ;)