Amy flipped through another page of her friend feed, her head resting on one arm. She could feel her eyes glazing over as picture after picture of someone's vacation scrolled past her screen, almost wishing that work was open today - at least it'd give her something to do.  The clock in the corner of her screen struck noon.
 
She switched tabs to 4chan, refreshing the catalog to see if any of the dozen boring threads she'd been following had been updated since the last time she'd checked it, ten minutes ago. 
 
"No, everything looks the same as the last time I checked," she thought. ”Guess it's back to dumb vacation photos.."
 
On second glance, however, everything wasn't different. In the middle of the page was a thread with a picture of a pastel-colored horse, the title reading "Pony porn thread? Pony porn thread." 
 
"Really? I thought these people had their own board now." 
 
Still, there was something enticing about the thumbnail, enough to make her want to click on it. As she did, the thread expanded, giving her a better glimpse at the picture in the first post. In it, a pony with light blue fur and rainbow-colored tail was presenting herself to the viewer, her tail raised high to reveal a soft, thick equine anus and a large, squishy equine vagina. Amy could feel a heat building between her legs, her body temperature rising slightly. 
 
The arousal brought her to her senses, and she quickly closed the window. "What the hell?" she asked herself, out loud. "Why the hell am I getting aroused by that? God, what would Matt think if he saw me like this?" 
 
"Still.. one more look couldn't hurt, right? I'll just show myself how stupid this whole thing is."
 
It only took a moment for her to find the thread in her browser history and re-open it. This time, she scrolled right past the first image and down a few more posts, until her eyes caught on another picture. This one was of a purple-colored pony being fucked from behind by a disembodied horse cock, an insert in the corner showing the thick equine cum flooding into the filly's womb. It wasn't until she saw the look of sheer lust on the pony's face - tongue lolling out and eyes wide open - that she began to realize how badly she wanted to see her boyfriend make that kind of face in bed. 
 
Almost without realizing it, Amy saved the picture to her laptop, and then scrolled down to find more. There was an odd stirring sensation between her legs, almost as if something was sliding around down there, but she ignored it as soon as she saw the next image. This one was of the rainbow-and-blue pony from the first picture licking the snatch of a second pony, white with a styled purple mane. The look on the rainbow pony's face practically screamed, "Stick it right in there, she's ready for you!"
 
It was at this moment that Amy's panties began to feel incredibly tight, her right thigh itching a little as well.  
 
"Ow! Dammit, I gotta get these things off!"  
 
She fumbled for the button on her jeans, which didn't seem to want to come loose. Amy looked down, and nearly screamed at what she saw.
 
Between her legs was a massive bulge, straining against her jeans and threatening to burst the button at any moment. The bulge pulsated and throbbed with her arousal, the movement visible through the fabric. Amy could actually feel a surge of growth flow through it, the button on her jeans flying off to reveal a massive pink organ attempting to tear her panties in two.
 
"Fucking god, is that.." She pulled her panties off, revealing a massive equine cock. It was at least as wide around as her arm, and easily as long as her arm from elbow to wrist, a thick ring of muscle around the middle. Dangling heavily underneath it was a pair of the most massive balls she'd ever seen, churning and begging for release. 
 
"I don't have a horsecock. I don't have a fucking horsecock." She was chanting it like a mantra, willing it to not be real.  "Mirror.. I'll just go to the mirror. It's not real." The sensation of the cock slapping against her thighs as she stood up from her desk chair was unbearable, and with each passing step to the mirror in the bathroom across the hall, she could actually see the cock leaking a clear fluid - precum.
 
When she reached the mirror, Amy almost wanted to cry. The mirror showed her mostly as she had been just minutes ago - a five-foot-eight twenty-something with auburn hair tied back into a ponytail.. only now she was sporting a horsecock.. and a strange tattoo on her thigh. A closer look revealed it to be a biological male symbol, in blue, penetrating a smaller biological female symbol in pink. As she watched, the skin around the dangling male organ began to loosen and envelop the base of the cock, with midnight black fur sprouting from it. Amy couldn't help but touch, stroking the soft fur and feeling how sensitive her new sheath was.
 
Her fear at turning into a horse subsided to a growing need to get off, her cock painfully stiff. It was certainly big enough that she could just bend her head down and suck, but something about that seemed to repulse her. "I gotta put this in something," she thought. "Where's some pussy when you need it?" She half-remembered the porn still on her laptop, and rushed back to it - pictures of pussy were at least better than having nothing to look at.
 
Amy quickly discovered that she couldn't fit one hand around her new organ, and awkwardly stroked it with both. It felt good, but just wasn't enough to get her off, her thoughts wandering toward warm, tight, wet horse pussy. The black fur from her sheath was rapidly spreading to cover her thighs and lower stomach, and a strange stretching sensation was starting to emanate from her forehead. Just then, another idea popped into her head. "It'd feel better if I could at least thrust a little. Wait.. we've got a spare pillow, right?" 
 
The closet in the bedroom she shared with her boyfriend yielded exactly what she was looking for, and soon she had her cock firmly stuffed between pillow and pillowcase. The pleasure was starting to turn to agony, the pillow just not tight enough for her. "Dammit, I wish I had something tighter!" she screamed, desperate for release.
 
With those words, a bright purple glow began to emanate from a point just in front of her forehead, nearly blinding her in the process. A beam of light hit the pillow, the case starting to shift, the cloth becoming thick and rubbery, tightening around her cock.
 
”Fuck! Fuck, I'm so close! Just keep on.." The pillowcase tightened more, the pillow itself wrapping around her cock. The inside of the pillow was textured and bumpy, squeezing against just the right places, each thrust bringing her closer and closer to release. The beam seemed to get stronger the closer she got, the pillowcase looking nothing like a pillow anymore and more like a gigantic cocksleeve. Amy thrust once, twice.. and then her cock began to spasm, shooting out gouts of thick white seed. A small lake of the stuff formed on the hardwood floor. 
 
Amy's arousal dissipated, allowing her to reach her hand upward and feel the long, spiralling horn that had grown from her forehead. "Not just a pony," she thought, "I'm turning into a freaking unicorn.. and it's awesom-"
 
Her thoughts were interrupted by her legs nearly falling out from under her. She'd barely been paying attention to the fur, which had now taken over her legs. Lying down on the floor, she quickly found what was happening. Her knees were inverting, becoming the strong back legs of a stallion, bulging with lean horse muscle, soft white fetlocks circling her ankles. She felt like she could run a marathon, but it was clear she'd be doing it on hooves. Her toes were already starting to merge and disappear, her feet looking less like actual horse hooves and more like the pastel-colored ponies she'd been jerking off to. 
 
She tried to get up, but found that the same thing was happening to her arms, forming into stumpy hooves that were still somehow dextrous enough to grab the sleeve off her cock - she had no idea how it worked, but it did. 
 
A few minutes later, Amy was strong enough to walk around on all fours, and slowly trotted her way into the bathroom again. She was barely recognizeable as human, her body having expanded into a form much more appropriate for a horse. The beginnings of a horse tail were already growing from above her rapidly expanding ass, her rear hole blossoming out and thickening until it was completely muscled and equine. She could actually feel her prostate gland churning away, producing another load of cum - a load that she was going to make Matt drink, whether he wanted to or not. 
 
Tail flicking, she returned to her laptop and saved all of the pictures from the thread, dozens of them. She barely felt her face push out into a proper equine muzzle with a thick, silky golden mane, completing her change into a very male unicorn. Turning the laptop to face the bedroom door, the images sliding by on the screen in a loop, she smiled broadly. Her boyfriend was going to make a perfect mare.. that much she'd be sure of.