You and Zecora have a little standing contract, one which gets you all the base price potions you could ever need. Twice a year, when her heat hits, you go to her hut and spend the next two weeks there.
 
Zecora's heat is a little bit.... different, than most mares; fitting, considering her equipment. While most ponies are driven by a desperate need to be bred, Zecora's heat is marked by the absolute carnal need to breed others. A need so powerful and so urgent that before she found you, she had to chained up inside her own home to keep her from raping and knocking up half the mares in Ponyville.
 
Fortunately, her lust-addled mind seems to take your more exotic form as a challenge. And so, you spend four weeks out of every fifty-two locked up inside an isolated hut in the Everfree, bent over and moaning as an unhinged, wild-eyed futanari zebra plunges her cock in and out of your ass with brutal force. She can't get you pregnant, but Zecora's instincts don't know that. Load after load of creamy mare jizz serves as your only lubrication, the the zebra unloading herself into you time and time again without restraint. Her rod, always so painfully tight inside of you when she starts, has long since destroyed any trace of resistance and slides in and out freely. Not that this makes the process any easier, as the the lessened resistance makes it only all the easier for the mare to thrust all the harder, her sweat-and-cum wettened flesh slapping against yours lewdly, echoing through the chamber. Your body rocks from the force every time she hilts herself fully inside, powerful horse cock easily forcing your body to make room for it.
 
Zecora's thrusts become shorter and shorter inside of you, the zebra increasingly reluctant to leave the soft, welcoming warmth of your body, and you have long since learned to recognize the telltale signs of her impending orgasm. As if on cue, the marecock buried inside you begins to flare out, forming a nice, tight plug to keep as much of her cum as possible deep inside of you, and even after all these times your traitorous body seems to -sing- in joyful anticipation of that glorious, soothing, fulfilling warmth. Above you, Zecora growls in possessive triumph, biting lightly at your shoulder instinctively to keep you in place, and the flood begins.
 
Despite how long the heat lasts, she never seems to run out, and the first long, hard spurt is just as voluminous as it was at the start of the week. Zebra jizz seems to be far less dense and thick than the pony variety - far more liquid and flowing - but this only allows Zecora to drive her load even deeper inside.The warmth seems to shoot through your entire body, completely enveloping you, and your body keeps faithfully taking her load no matter how many times those balls twitch and pump.
 
Zecora keeps you there, possessively draping her body over yours, keeping you still and and pinned until she is confident you have taken every last drop, until she finally pulls free of you and your noticeably-larger belly with a  satisfied sigh. Her flare is rapidly softening out, but still easily large to prompt a -pop- as she tugs it free of your slightly-gaping asshole, now colored as white as as the lighter portions of her fur.
 
Momentarily sated, the zebra's body relaxes, slouching into you. She pins you now with weight rather than muscle, your mistress seeming perfectly content to use you as a makeshift bed. Her dark black mare cock retreats back into its sheath as her eyes flutter closed, heavy balls already churning up a fresh load of zebra baby batter to replace what she had just spent on you.
 
It would be three or four hours at the longest before she woke from her nap, powerful mating instincts once more sensing the siren call of a warm, soft, welcoming form nearby. From there, it would be only seconds until she once again registered you as not properly impregnated, and milliseconds until her stallionhood demanded that mistake be properly corrected.
 
Four days down; ten to go.