I'd have to agree with marvin's answer. If the simple contact with air is enough to form highly acidic substances, your creatures, assuming them to work exactly like vertebrate life on earth can't really:
Open their mouths (which are damp since saliva has water. This also means panting like a dog to cool down will likely be an unpleasant experience).
Breathe (most of the respiratory system is kept moist at all times).
Eat (small amounts of air do enter your digestive track during the eating process, meaning this air could potentially turn the moisture all the way to your stomach into acid, which is a problem).
Get hurt in any way which resulted in decent bleeding (at least in humans, plasma is responsible for over half of our blood volume, and plasma is itself 90% water).
See with eyes like our own (our eyes can capture light because they have many living cells, and because they're living they need to be kept moist).
Smell with noses like our own (we capture smell particles mostly thanks to the mucus in our nose, which is also meant to help moisturizing the air before it enters the lungs. The snake method of smelling also doesn't work, because the tongue and the vomeronasal organ are kept damp as well
To sum it up: vertebrates like those found on earth as we know them would, in their majority, have little to no chance of survival, because each and every one of them had their evolution focused around one thing: water. Every single one of them came from the ocean, and adaptations from land are basically separated into 2 groups: moving on land and conserving water inside their systems, but we have little to no adaptations to dealing with said water suddenly turning into acid as far as I'm aware.
If these alien mammals can survive in your world's conditions, that means they don't have to worry about the acid at all, because unlike our world's mammals, your mammals are more than adapted to these acid environments, thanks to the billions of years of evolution selected by said acidic environment. This also accounts any and all: reptiles, fish, birds, fungi, echinoderms, plants, mollusks, arthropods, annelids, euchariote cells, bacteria and any other organic life your world might have.
There's only one scenario I can see in which we have mammals not adapted to the harsh environment suddenly appearing in the planet without any trace of being adapted to it: a group of very stubborn humans who traveled to this world in hopes of colonizing it and yet decided to stay despite knowing that every single time they leave their airtight, filtered stations, they'd need to wear airtight, hamzat-suit style protection suits with self-enclosed breathing apparatuses, and that if said suits or living spaces suffer a breach, the people inside them have a good chance of dying.