Jessica giggles as your fingers work their way around her sensitive waist. "That tickles, Johnny!" she laughs, swatting your digits with her hands on the outside as your work your arms further in and around. "And your elbows look all lumpy under there wrapped around me... here, how about sliding your arms down the front of my thighs? Maybe it'll just look like extra horse-leg-muscle that way."
You do as asked, and realize that positioning actually anchors you a bit better - your shoulders are braced alongside Jess's hips and your hands reach down to grip her knees. It feels, sturdy and ... more natural this way. The top of the furry opening is scrunched between your face and her back, so you twist your head to get it inside the costume. Jessica reaches back to help, and soon you're encased in darkness. There must have been velcro or more elastic around the edge of that opening, because you can't see even a glimmer of the dressing room lights anymore. Surprisingly, it's not stuffy; you find you can breath perfectly well, though the smell of your girlfriend surrounds you. Heaving a sigh, you rest your cheek on her lower back, your chin grazing the swell of her ass. This doesn't feel as awkward as you thought it might.
"Oh that looks great!" you hear Jess clap, and you can feel her bending one leg and then the other, shifting your shoulders and arms as well. "I wonder what we'll look like walking? Let's try, John! Follow my lead!" she begins to turn slowly, and you carefully follow, the costume's hooves clopping on the linoleum floor. It's warm in the suit, but not uncomfortably so. And your girlfriend smells so good: sorta like that cocoa butter lotion she uses, and something... earthy. You rub your cheek against her back, nuzzling into her soft skin.
Meanwhile, Jessica is entranced by the costume's realistic appearance! Her head swivels as she watches the creature they've become turn in the large changing room mirror. This is not like her parent's saggy horse costume at all! Johnny's rear end has the true, rounded appearance of a horse's behind, smooth muscle rippling beneath the lustrous brown fur. She notices for the first time that there's a ropey tail hanging down - different from a horses tail of long hairs, this one is more of a fur-covered appendage ending in a tuft of longer hairs, like a lion's or...
"It's a unicorn tail!" she exclaims. "Johnny, we have a unicorn tail hanging down in back! And it's swaying realistically as we walk!" she hears her boyfriend grunt in acknowledgment, and feels him continue to rub his face against the top of her butt. It feels... good. She's tingling there, probably from his light scruff of whiskers. "Mmm, babe... that feels..." His nose nudges at the waistband of her panties, pushing it down and sinking a little between her cheeks. The tingling spreads as she feels his rough cheeks and chin graze her sensitive skin, spreading her as he pushes his face in further. "Ohhh, Johnny! ...John!" she whisper hisses, remembering they're in the changing room of a public store. His nose is pressing against the base of her spine, and she feels a pressure building as he pushes in farther and farther. He doesn't stop. It's as if he's melting into her, and the sensation is extremely pleasurable!
Pressure and pleasure build higher and higher, and Jess isn't sure how she remains standing except that the pressure seems to be squeezing inward on her hips and thighs as well, tingles racing down her legs and back up to center in her tailbone. They've never done anything even remotely similar in the bedroom, so the sudden explosion of her orgasm racing through her catches Jessica by surprise, right as she felt the pushing pressure release with a physical *pop*!
Hands braced against the mirror, Jess pants through the aftershocks, feeling that Johnny has stilled as well. His ribs are heaving in time with her breaths, and strangely, she can feel them inflating just behind her legs. She raises her head, the tip of her unicorn horn grazing the mirror on the way up and as she catches a glimpse of her reflection, her jaw drops open in complete wonder.