Generally speaking, authentic Thai food is not vegan or vegetarian. It only will be if it was specifically prepared as such, but this is not usual at a Thai restaurant.
Typically, Thai food is served with meat or fish but any Thai dish can be ordered vegetarian style easily if you know how to speak a little Thai.
“Pak” is a Thai word for vegetables, and if you substitute that word for the meat in the Thai dish, they will understand that you are requesting a vegetarian meal. If they still seem confused, you can also say “my yow nua” which means that you don’t want meat.
Thailand is a Buddhist country, and there are some Thai vegetarians here, but I wouldn’t say that it’s prevalent. They eat a lot of meat in Thailand, especially pork. But you can get a vegetarian dish by request at any Thai restaurant.
If you are vegan, it’s a little more tricky because you might have to worry about some of the sauces that they use in Thai food. Fish sauce is one of the most popular flavorings for Thai food, for example. But, if you’re a vegan I’m sure you’re used to navigating these obstacles when you are eating out.