thaaat. is a good question. that i definitely know what means. i am in fact one hundred percent sure this is the response u want:
lore/headcanons below cut, i've color-coded it a bit cuz i have ADHD and if I ever need to read this again my brain *will* shut off the moment i see how many paragraphs are in this. (sorry if the colors annoy you, lol)
Okay so, first of all i wanted Horrorswap to be dealing with a different problem than starvation, so in this one it's air pollution. basically everyone's struggling to breathe (this included the skeletons, because in my personal hc, monsters are made of the same stuffs, thus it doesn't matter if ur a skellie or a fish or a horse, u breathe the same way as everyone else... but then i was like... "actually no, i have a different idea")
So, Reach using toxic gas (he's got both flammable and non-flammable ones) and Goose using fire (which produces smoke or makes Reach's gas explode) seems a pretty good way to weaponize the fact that almost no one else can friggin breathe down here.
Then comes the question, *why* exactly are they trying to make living conditions worse for everyone else?
Well, overpopulation, dwindling resources, the works: They've calculated how quickly the underground's gonna start going to shit if they don't get that stuff under control and have chosen self-preservation over anything else (the opposite of my Horrortale au, Wretchtale, where all the monsters are doing everything they can to stick together)
But why did air pollution become the problem first, over something else?
I'm imagining their magic technology was more gas-based than heat-based in this universe, like maybe they didn't have a Hotland, but instead some sorta toxic/gaseous area instead, which i think'd be swag. it also means there'd be a decent amount of monsters who *are* built against toxic gas and maybe also smoke, which is where Reach and Goose use their magic instead. Idk, it's obviously all still in development, i literally *just* made up the au like, 1-2 days ago xD (for now we'll change the name of Hotland to Gasland cuz that's about as unimaginative as we can make it)
Ok, so why in the world does Reach wear a gas mask, if he's completely fine to not breathe?
Well, here we'll be going with my personal skeleton monster hcs where i don't actually draw them like skeletons: they've got ribcages and spines and all that junk, but they've also got "fat" (I'll be calling it that cuz it mostly functions like human fat would, but technically it's just magic) and depending on how much magic food they eat vs how much they expend, the magic will pool in the areas that make most sense (viewn from a human standpoint with where fat ends up) so they've got most of it pooled around the spine, it also thickens their bones and comes from beneath the ribs to, at some point when humans would be considered "fat", cover over the ribs as well (which is why you'd see the ribs on Gooseberry who works out and uses more magic since in-fighting is a more regular occurence in the Horrorswap underground, but you wouldn't see Wretch's ribs cuz he needs to keep as much magic stored up as he can to stay functioning, pretty much.)
Anyway, so to answer the question, their anatomy is still more related to humans that skeletons (as much as a magical monster can be compared to a human), and as such, they're more like... "Resistant" to the air pollution, rather than "Immune" like some other monsters would be. And while this is mostly fine for Goose, Reach used to have a bad smoking habit and has developed Asthma. Good job Reach, you dumbass. Anyway he's usually fine, but when him and his bro are doing gas and smoke stuff, he can't be having a shitty coughing fit in the middle of it, and thus the gas mask comes into play.
As for other monsters in the underground...
Obviously with Gasland being a place, there's available gear to deal with the air pollution, but there's also not enough resources for everyone to have the luxury of gear like that, and thus it's mostly prioritized for people nearer the most affected areas, + the most important monsters, like the royal guard etc.
The number one priority of the underground is getting out, of course.
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Aight I'll end the post here, but if anyone else is curious about my Horrorswap au, that I'm now naming, uhhh... Vaporswap, which took me 5 minutes to arrive at because every idea i had, was somehow already taken. god we need to stop making so many aus...
more likely, he'd set u on fire. but it's okay, u can carry him around if u want uwu
imma try to add at least one fact to any ask like this, just so it's not *just* a random silly.
HCs/Lore below
In Wretchtale, I decided Mettaton should be the ruler of the underground, which means in Vaporswap, it's gonna be Napstaton instead! Huge plus considering robots/ghosts don't care about breathing!
In Wretchtale, Knave (Wretch!Papyrus) works for Mettaton, and is head of the royal guard, but Gooseberry isn't in the royal guard in this au. He's too busy wrecking people's shit.
3. Goose spends most of his time hanging out at the Head of the Royal Guard (Alphys)'s fort in Gasland
