I got this halter to try on my 13 hh pony. The size listed was "Large Pony/Arab". It is honestly a little big for him. So if you have a petite faced pony, I would either double-check the measurements (customer service is great), or go with a smaller size than you think. I even got it to fit on my 15.2 hh TWH. The pony did wear it under bridle for a 25 mile endurance ride and did not bulk or seem uncomfortable to him, even at the end of the ride. I didn't get a chance to try it under a driving bridle, as my cart is currently out of service, but i imagine it would not get in the way. Overall, a wonderful product. Just check measurements.

This halter is thin enough that works great under any type of bridle for out on the trail (especially bitless), but it is a bit thin and so curls a little. That being said it does not affect the use, just the ascetics of the halter.