“What an odd contraption.” 

Rarity nudges the clear rubber tubing with her hoof and sneers with disgust. 

“I hope you realize I paid twenty bits for this thing! Its very existence appalls me!” 

Anon surfaces from smooching Rarity’s rump with a giddy smile on his face.
“Well, you said you didn’t want any unpleasant odors distracting you from your work, right?”

“Yes, I suppose that’s true...” 
She looks to the box; the crude graphic designs with big yellow fart clouds spelling out the name Windhaler 6000.

Just as she’s sizing up her options an impatient gurgle sounds from her stomach. 

“Ugh. Well, I suppose there’s no time to dwell on this. A-Are you ready?” 

Her expression is an uncertain one; she catches her lower lip under her teeth and lifts the mask into the air with magic.

“Here goes nothing. I just hope you know what you’re getting yourself into, dear.” 

The mask presses into Anon’s face and the straps tighten by themselves. He reclines into a heap of perfumed pillows and watches the other end of the tube levitate towards Rarity’s fat marshmallow cheeks.

She pauses and scowls at the vulgar device before bracing herself and plugging it in place with a plop, nestled tightly between her flanks.

“Oh, this feels obscene!” She shudders at the presence of the hard plastic nozzle in her butthole, but tries to distract herself by turning to the wooden pony mannequin in front of her.

It’s mounted with a few loose strips of fabric that she quickly gets to work on.
Her scissors make little incisions here and there, she weaves a needle along the outer edge and pulls the whole thing together into a pleated-

DEAR CELESTIA ABOVE

The fierce smell rushes into the mask without a sound. Anon’s instinct is to cough and splutter with genuine surprise.

Expired broccoli and sour sulfur sting his nostrils as he huffs the sweltering fumes. 
He wasn’t prepared for Ponyville’s premier proponent of all things fashion to have such abysmal-smelling farts. 
And yet, as objectively repulsive as it is, his cock couldn't be harder.

It’s her diet, and largely the reason for her agreeing to fulfil this strange request of his in the first place. 

She sticks to a strict regimen of steamed vegetables and whole oats, and these healthy meals come through her system smelling like the deepest, dampest recesses of a sulfur mine. 
Today's intake was no different; a great big heaping pile of asparagus and broccoli for lunch, a tall glass of oat milk to wash it down, and a zesty cheese quiche for dinner. The two glasses of plum wine from Vanhoover seemed like a nice idea at the time, but she always finds alcohol and dairy to be a quite tumultuous pairing.

After a few more sniffs the gas has mostly cleared from the mask, and Anon takes a second to recover.

But a second is all he’s given before Rarity issues a dainty, ladylike grunt and sends fresh fumes hissing through the tube and flooding his confined breathing compartment. 

He tries to sniff these too but his nose struggles to keep up with the stench. His face is growing sweaty from exposure to the heated gases and the little visor that allows him to see outside is fogged with condensation. 

He struggles, trying to grunt out some submission to Rarity, but she shushes him.

“Anonymous, really! I’m trying to focus here, could you please keep the fidgeting to a minimum?” 

He’s in a kind of turmoil; on one hand, his cock is positively palpitating at this excess of raw flatus. On the other, he’s growing more and more lightheaded. Turns out pure equine flatus isn’t a rich source of oxygen.

Rarity makes some modest adjustments on the bed, lowering her head with a long growl to push out the big gas bubble that’s been bothering her since lunchtime and snowballing all afternoon.

All Anon hears is a sinister whispering sound as rank air flushes into the helmet. He takes one sniff and already it’s worse than the last two farts combined. The fruity kick of alcohol has now joined in on bullying his nose and testing the limits of his arousal.

After ten seconds the searing blowtorch of hot wind ceases, and Rarity sighs and closes her eyes in a moment of bliss.
"Mhh. As thoroughly indecent as it is for a lady to pass wind, I can't deny it feels rather nice!"

Just a few feet behind her the overzealous human is broiling away without a single waft of oxygen to keep him going. 
Muffled cries come from the mask. His body struggles on the bed, and then he falls limp. 

“Darling? Anon?” 

She peers back to inspect the other end of the gas tube, and finds a passed-out ragdoll of a man.

“Oh, goodness.” She raises a hoof to her chin. 
“Perhaps I ought to lay off the asparagus.”