Lets be honest, you weren't expecting this.
The costume in your hand, the one that won't let go, is a red bodysuit, bright crimson really. It's shiny and the entire thing squeaks lightly in your hand as you grab it. It's slick, the entire thing seemingly made out of a thin latex-like material. Thankfully, perhaps, the head seems like it'll be uncovered and you're under no compulsion to grab anything else.
Finding yourself speed-walking to a dressing room in back, your body allows you to take a moment to examine the costume. It's...again, not exactly what you thought it would be. You can see little dainty hooves at the feet, a long whip-like tail at the back, and, worst of all, it's clearly meant for a girl given the small tear-drop shaped breasts and the flat crotch.
"...seriously?! This?!" Disbelief coming through in your voice as your body, on autopilot really, begins to strip out of your clothing, leaving you naked with nothing but the latex bodysuit a few moments later. Where your clothes went, you can't really tell. Hopefully they're behind you but you can't see them in the full body mirror that's along one wall of the dressing room.
Your face reddens slightly, quite a few shades less red than the costume itself, but still a flush that's visible. It's made worse by the way your little buddy is beginning to poke itself free, rising to length as you slowly feel over the costume. "The fuck?!" works its way out of your mouth as you stare down at both the costume and the visible sign of your arousal.
Your body sits, undoing a zipper that you could swear wasn't there a moment ago, unzipping the costume all the way down the back, almost splitting it in two. With a dainty movement, your feet slowly begin to slide into the thing. It's...slick and slimy. Oddly warm too, as if a living thing almost. "T-This is really weird..." trying to raise the panic and disgust you feel into your voice, but it seems like whatever force controls your body is making sure you don't start screaming.
And then it hits you. A shock of sudden pleasure as your left foot slides into the hoof. You'd expect that it would bulge out, show your clearly masculine legs. Instead, all that's visible is a slender hoof and the feminine leg that goes with it. As your eyes wander upwards, you notice, to horror how your knees are bent the wrong way and how you can't really feel your toes. As soon as it hits though, another shock of pleasure hits you, the costume seeming to adjust itself up to your left thigh, showing that same slender leg that started with the hoof.
The right leg soon follows, a calming warmth and pleasure starting to emanate from the covered places. Somehow, you...you're less and less concerned with how scary this is. Sure, it's terrifying but...it feels nice.
"I'll just take it off when I'm done, yeah..." your voice rationalizing. Your mind thinks this is probably a good idea. It's just a costume. That's it. Why not enjoy it?
"Yeah, I'll enjoy it" you say to yourself, standing up on your new hooves, covered in monochromatic and shapely crimson latex up to your thighs. Your legs visibly look longer, at least in proportion though you could swear you're shorter.
That thought seems to leave you as you, you think it's you, begin to pull the costume up, past your pelvis and up to your belly button. The tight latex, warm and sticky and soothing, pushes up against your erect crotch in the most lovely of ways. You look down to see the bulge, but instead, only flatness that's slowly, somehow, working your groin properly. The engulfing warmth is better than any girl you've been with, tight as can be. You barely even notice when you gush out, coating the insides of the costume. Not that you could tell really, your release having been consumed, your bulge feeling gone, but that lovely sensation of pure pleasure hasn't left yet.
Your tail, a new addition, whips around lightly, slowly working itself around a leg. It's a long thing, almost four feet, two inches thick and strong, with a lovely spade at the end. It, and you swear you're not doing it, teases at your groin, slowly working its way inside your new opening, sending more fireworks into your brain. "A-Ah?!" You manage to gasp out, arching your back as the costume slowly seals itself up around your waist, showing off a thin one without a belly button. It's lightly muscled, slender and quite pretty, matching your girlish hips and legs.
"H-Hells B-Below...t-that's..." You don't notice what you said, of course. You're too busy to, wrapped up in this almost unholy pleasure. But...it feels too good, why should it be bad? Shouldn't everyone feel this good? They should, shouldn't they? You nod absently to these thoughts as your arms begin to slip themselves into the costume. It's up to your rib-cage now, to the moment of truth as you lean forward slightly thanks to a skilled tail-wiggle, voice coming out as a moan.
And then they strike! Your breasts attach themselves to your body, pleasure now shooting into your brain from two more locations, currently unmolested but that will be changed soon enough. Cute perky nipples hang themselves off your cute perky breasts, not too big, but certainly not small, perfect sized really for...for whatever you're becoming.
An arm slides forward, followed by the other. They're eager, you can tell. Eager to finish? Eager to massage your new breasts? Hair sliding off as they extend into the crimson arms of the costume, muscle and fat fading into slender little things. Your hands fit perfectly into the costume's, sharp dainty claws, again that same color as the rest of you, replacing once dull nails. "A-Ah...more...please..." you're begging now, caught in the throes of passion as your tail plunges in, newly formed hands teasing at breasts.
"M-More! More! More! More!" A chant, a refrain, coming from your mouth as the costume begins to cover shoulders, slimmer and perfect...and finally your neck, zipping itself up higher and higher until the zipper itself poofs into nothingness. "MORE MORE MORE MORE MORE!" yelling, screaming, voice massaged and tended, deep to high, serious to sing-song, male to very very female.
"YESSSSSSSSSS!" you scream, body releasing in one gigantic spurt, all pressure and tension and release gone. And then you're alone, in the dressing room with a mirror to keep you company. Your body is your own again and you look and see what's become of you. What do you do?