We had a mare present this way this year. Her 4th foal - she was not pushing much and when I gently pushed in to check position (sac was showing) I could tell foot was upside down, and it was almost trying to perforate the rectum actually. I went ahead and guided it out so the upward force wouldn’t cause a tear.
The foal seemed to be in the right position other than being upside down. Called the vet, and she did not seem that worried (this was our first upside down), so we tried not to panic. Mare got up and down 3 times and low and behold, the foal turned. It was amazing to me! We are used to them just spitting out in like 5-10 minutes so it was worrisome, but obviously our vet was right not to be overly concerned. Filly was born healthy and fine with just a little help on our part for the shoulders.
Very pretty foal! And yes, big!!