First,
Congratulations on a healthy live filly.
Second,
Before all of you run out and buy fish tank hose, here is a word of warning...... It is my understanding with over 40 years of raising animals, that the puppy, foal, human baby, etc. does not start trying to breath till the placenta is no longer attached. The baby gets its oxygen from the placenta and when that is either detached from the uterine wall, or the cord severed by birth, or pinched in the birthing canal, the baby does not try to breath. The chest wall has to be free to expand in order to draw in a breath.
The 1/2" fish tank hose, though flexible, is still capable of tearing a lung or going into the stomach rather than the lungs. (plus it is very large for a newborn foal) You can also introduce bacteria this way, into the lungs. Plus, when pushing the foal back inside the dam, how would one keep the tubes in the foals nose without causing damage? The air that may have been heard passing, is probably air that escaped out.
Vets, especially young ones, still have trouble being able to tell if they have the tube in the lungs or stomach when trying to tube a Mini for colic. They have to feel the neck as the tube goes down, then test to make sure it is in the stomach. How is an inexperienced person supposed to be able to do this? Someone who is new or relatively new in foaling may cause some damage.
I am very glad that the filly made it out alive, but, my guess would be that the placenta was still attached to the uterine wall, and that is why the filly lived.
I am only posting this so that others do not run out and try it themselves and perhaps cause other problems. Your foal are precious to you, please be careful.
I am not trying to put anyone down here with their ideas, but I'm just concerned that others will run out and try this, and it may do more damage than good.