"Emily, for as far back as you remember, you've always been deeply aroused by horse cock."
Emily gasped and blushed as she looked away from you, ashamed. On the wall behind her, you can see the perfectly innocent horse poster change into a much more... Suggestive image of a stallion rearing back, muscles clearly defined and the outline of its majestic for against the setting sun made it very clear it was a male made for breeding, clearly highlighting the size and girth of its sex.
On Emily's bedside table, a few books shimmer into existance, tagged by the school library, with titles such as "Animal Husbandry 4: Goats, Rams and Stallion" and "How to draw Horses: A study of Anatomy". Under her blanket, you can see the tip of what seems to be a faithful reproduction of a horse cock, albeit at a slightly more manageable size, that glistened with Emily's juices, the dildo an obvious giveaway of her new deviancy.
"Y-you noticed, huh?" She asks meekly, pushing a strand of hair behind her ear. "I'm... I'm sorry. It must seems weird to you."
You grin, loving to see your debauched little sister ashamed of the life-changing kinks you just gave her without her knowing. "A little, I'll admit." you reply. "I mean, I knew you were always a bit of a horse-girl, but I didn't think you'd be wet at imagining being taken by a stallion."
She squirms in place, fighting the arousing thoughts that well in her at the image you just suggested to her. "N-no, that's silly. Even if I wanted to, horse penises are much too large for my... I mean, for any woman's sex. But even if I wanted that... So... So badly, I'm not reckless enough to try it."
You nod. "A shame. So when did you start creaming your panties about horses?"
Her cheeks grow bright red. "Uhm... Well, remember when mom and dad took me for horseriding lessons? When I got there, the lady let me go in the stables and meet the horse I'd ride while she discussed payment with mom. I was supposed to go to the box of this older mare, but, I wandered too far and hear whinnies so I went to investigate.... And... " she bites her lip as she squeezes her thighs together, obviously caught in her own arousal as she revisited the naughty memory." there was a stallion and a young filly... It smelled strong of musk and the stallion was... Very ready. His cock was... Oh god... It was so big and veiny." she says, panting, spreading her legs to let her hand trace the outline of her slit through her jeans, utterly incapable of stopping herself despite you being right there.
" The filly, she must have been in heat because... God... Because she was whimpering and bucking and the stallion climbed on her and I was just... Fascinated! I looked at them while he penetrated her, inch by inch that went into the young horse I was quivering and trembling.... I wanted to be her. I wanted to be bred too! I wanted to hug his big, thick cock almost as big as me and jerk it until it disgorged all that thick horse semen onto my nubile little body, li King it clean while I fingered myself and.... And..."
Her eyes snapped open and she immediately clenched her trembling thighs together, gasping at the shameless display she was giving you while delving through her deepest, most arousing fetish." Oh god... I'm so sorry! I... I don't know what came over me. I...." she looks away, trying to hide her face from you. "I guess I really am a filthy pervert huh?"
Your cock is straining against your jeans so hard you feel you're about to burst. "Yeah..." you say through shallow breaths. "Yeah you are... "