You're a scientist working on a centaur breeding program. They're extremely endangered because only males occur in the wild, and they need human wombs to reproduce. Not many people want to carry a foal, but you've nearly perfected an artificial womb.
You've captured a male, 15 hands high and so deep into his heat he can't be reasoned with. You have all six of his limbs shackled, and every hour you inject more hormones into him to keep him hard and aching. It's physically painful for him to go longer than an hour without ejaculating, but you're a good doctor and you milk him every time he starts to bellow.
He's bellowing now, the straps that keep him in place creaking. His two foot cock twitches to life, thickening as you watch. Your eyes lock with his, and you see desire in his eyes. "Please," he growls.
"Just a moment," you tell him. You need to collect another vial, but there's none in the cupboard. He's already filled all four gallon jugs, and his heat hasn't abated.
"I need to fuck," he pants. Already his cock is dripping with precum. "Let me grind," he begs. He shifts from foot to foot, hooves clattering against the floor. You ignore his fidgeting and cross to the supply closet.
There's a crunching noise as he tears his restraints from the wall. You whirl around, and he's already on you. His hands lay you out on the exam table. He shreds your lab coat, yanks your pants down to your knees. His breathing is harsh and loud in the room.
"Please," you stammer. "I'll get you off, I promise--"
"Too late," he growls, and rolls you over so your bare breast press against cold metal and your cunt is exposed to the room.
"It won't fit!" you cry. You've been the one milking him all day, you know intimately that he's larger around than a pop can, that he's as long as your arm. But he rears up and positions himself over you, his hairy barrel chest pinning you down, and then he's inside you, bruising your cervix in one thrust.
His groan of pleasure echoes in the room. "That's good," he moans. "Nothing like a frigid boy's cunt to get off on."
You bite your arm to keep from crying out. Your cunt burns from the stretch. It's impossible that he's inside you. It's even more impossible that it feels good.
He jackhammers into you, wrecking your cunt for anyone else ever again. His grunts are the only thing louder than your hammering heart as he forces himself past your cervix, locking himself inside of you. His cock throbs as he comes, rope after rope of seed filling you up. It's more than your womb can hold, and your belly presses into the table.
He softens, and you shift your hips, desperate for freedom. Cum runs down your legs, dripping onto the floor.
"Please," you gasp. "Please, please--" you aren't sure what you're begging for.
His cock twitches inside you. "Beg more," he growls.
He half pulls out, and then thrusts back in, already hard again. "Too bad," he says. "You said you'd get me off, and I'm not satisfied yet."
He cums in you five more times. You're a shaking mess by the end of it, so overstimulated that every brush against your skin is torture. Your tdick throbs. You've been on the edge of orgasm for an hour, and he knows it.
He pulls out finally and rolls you over. Your belly is so distended you look six months pregnant. He lifts your knees in a mating press, even though your legs are coated in his slick. He bends his face between your legs and licks across your dick, then takes it into his mouth and sucks it. You cum so hard your vision goes white, but you can still hear him as he croons, "That's right, take all of my seed. You'll get your foals, don't worry."
