The coldest we had here so far was a brief 22F and it did not damage the plant, but it was kept dry. I think as long as the plant has already managed to form a hard bark it is able to handle the colder temps. But if your plant still has a green caudex, better bring it indoors.
I have also learned to move some plants much closer to the house when there is a frost advisory. Then particular to ponytail palm, I make sure to move it back to the sunniest side of the garden the following day and keeping it dry if possible. Our mild winter is more on the rain side, so my ponytail palm has lots of pumice in its media too, so if it happens to rain, it always drains fast.
