To most ponies, the small little click of a closing padlock didn’t mean much. To some, it meant that their diary was safe from prying eyes; to others, it meant that they could be sure that their gate would remain closed throughout the night. It meant so much more, to Essa Bolt, though…it meant a job well done, another life made more secure, and the world made just a little bit safer.
 
Well…usually, that was the case. Now…every single click made Essa shiver, and sent a wave of nervous pleasure down her spine. Each lock sent out an almost-deafening (to her ears) noise as it clasped shut, trapping her even more in her current outfit. She happened to be wearing one of her favorite latex catsuits, with an integrated network of tight, grippy straps, and even a saddle. The rubber seemed to somehow grip tighter with each padlock that snapped shut around the clasps of each belt, sealing her even further into helplessness. First the chest, then her forelegs, then the saddle, then her flanks, and finally…the tail wrap.
 
There was just one lock left…the one that would seal the latex muzzle shut around her snout, and cut her off completely from breathing. For the moment, this strap was undone, but the muzzle still hung loosely across the bridge of her nose…the scent of rubber was strong, seemingly brushing against every single smell receptor she had. Even now, every breath she breathed out returned to tickle her nose with warm currents. Soon…there would be more to this interaction. Every inhalation would only stretch the latex tightly against her lips as it protruded into her maw, coating her tongue, and suctioning itself as far back in her mouth as it would go. Her lungs would burn and heave as she tried to breathe even any stale, expired air, but would find no more to spare within the tiny pocket she was afforded. Every exhale would be bitter, pushing the latex out, removing its taste and comforting hug, but its scent still yet remaining as the muzzle would billow out, tugging on its base, inviting her to inhale once more…and to begin everything again. Over, and over, and over…
 
Click!
 
Essa hadn’t even realized she’d spaced out…the belt was already tightened, and that click was the signal that her last free breath had already passed. Now came the fun part…being completely helpless, locked into her suit, and yet otherwise free to move as she wished. As the first inhalation sucked the latex firmly against her teeth and around her tongue, she felt a tingling between her haunches, and her heartbeat getting even faster. Already her lungs were shuddering, both from anticipation and the, to them, unexpected stretchy, yet still-unyielding barrier.
 
She knew the lock dangling from her chin wouldn’t be undone until she slipped into unconsciousness, fighting the rubber the entire time. She knew that while painful at first, near the end, she’d be overcome by lightheadedness, pleasure, and leaden tiredness all at once, before everything would go black. Then she’d wake up, free to drink in all the cool air she needed. That was a great benefit of doing breathplay with a friend. But until that all happened…
 
“Hhhhhhhg…! Hhhgghghhh…! Ghhhhhn…!”
 
Gasping desperately for air would be the only meaningful thing she could do.  
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Pink/blue latex alts in the source description!