I have three horses and every once in a while they get a cold. Horse snot is pretty bad but sometimes they make it worse by rubbing their noses in the dirt and it turns into muddy snot.
I think I can help your pony for real. I have learned that Castor Oil does magic for horses. I would take pony and get myself to anyplace that sells castor oil and get a lot of cotton batting and something to wrap around pony.
I don't care if he is a stallion. If he is a stallion, he is not a pony.
Unless pony means something in horse that I don't get.
So, the pony gets the castor oil treatment. I would love some heat. I would love some plastic wrap.
Nature would do the rest. It will decongest pony and release toxins in his organs safely. Meanwhile pony may be breathing something toxic in the air in which case we are all gonna die so the question is moot.