(Moving this over manually from Derpibooru because it seems to have been overlooked by the automatic importer and I am impatient. Original description follows.)
Image from Horselover Fat’s collection of AI images. Slightly edited to fix some obvious bad anatomy.
TG/TF story below:

“Mike! Hey!” a dainty, bird-like voice called out from behind. Mike turned to see something he’d never expected. There was a girl running toward him, and she was hot. Not just hot, but straight up beautiful.

Shimmering blue eyes locked onto him, but his were busy traveling up and down her running figure trying to force it all to make sense. She was tall, leggy, had really wide hips, a slender waist, two almost spherical breasts crammed into a tight blue dress jumping around with every click of her crystal-clear plastic heels on the mall floor as she closed the distance between her and him. Thick masses of blue or purple hair with glistening sparkles billowed behind her as it tried to keep up with her body.

Mike realized that a lot of what he’d thought was hair was actually a large, full-bodied horse tail. A big, fluffy ear flicked in excitement when she finally got close enough to slow down. There was a bone-white horn spiraling out of her forehead. Her entire body was some sort of slightly pastel purple-white color. It was an odd combination of beauty and unreality that somehow seemed perfectly, disturbingly natural together.

She waved an arm as if to snap him out of a trance. “Earth to Mike! Yoo-hoo! Wake up, Mic-Mac!”

He blinked, jarred out of his suspended consciousness by the use of that familiar moniker. “Huh? Did Brandon send you to get me, or something?” he asked, wondering how this unearthly creature knew his friend’s private nickname for him.

“Uh, did I send me to get you? What are you talking about, dude?” she asked, just as confused as him.

“Wait, hold up. Brandon?” he asked.

“Who else?” the unicorn girl replied, like someone had just asked how many heads she had on her shoulders.

“Dude, why are you a unicorn chick?”

“Why am I a … what… huh?” the stranger stammered for a moment, before looking down at herself. An expression of realization slowly crawled across her delicate features. “Wait, wait a minute, wasn’t I…?” She held up her hands and looked at them studiously. “No, you’re right, this is different…” she said after some consideration. Then she started turning and twisting to check more of herself. “Huh, weird! I remember being a dude before, but, like… I dunno how this happened.”

“Okay, well,” Mike said, scratching his head and trying to look at something other than his friend’s enticing new cleavage. “Just try to retrace your steps. You woke up normal this morning, didn’t you?”

“I mean, this feels normal,” she said, tugging on her dress. “But I definitely remember wearing pants when I left the house today.”

“Okay, okay, that’s a start,” Mike said, encouraged. “Anything weird happen between then and now?”

“No, just hung out online like usual, then remembered to meet you here so we could get the latest expansion packs at the game store. Uh, I parked on the east side so my car would warm up in the sun, so I had to go through the Stacy’s department store. They had some areas blocked off for redecorating, so I had to go through the ladies’ section to get out of there into the mall-”

“Holy crap! Brandon, you didn’t!” Mike blurted out in shock.

“Didn’t what?” the feminine unicorn asked, worry seeping into her voice at her friend’s tone.

“Didn’t what? You idiot!” Mike reached out reflexively to shake his friend back to sense, but barely restrained himself. “You walked through no-man’s land! NEVER go through the ladies’ department at Stacy’s! What were you thinking?”

“Oh! … oh!” she gasped, her face becoming ashen as if she’d just learned she’d strolled through a minefield. She looked down at herself again. “Oh crap.”

Mike sighed. “Okay, so… what now?”

“Well I still want to get those new expansion packs,” she said after a moment.

“Okay,” Mike said, “and what about after that? You need to, like, buy some new clothes or something?” he asked, gesturing vaguely up and down his friend’s new body.

“Probably, but I only have enough for the cards. Hey, Mic-mac… I don’t suppose you would mind…?”

“Oh no, no no no, forget it!” Mike said defiantly. “Geez man, you’ve been a chick for thirty seconds and you’re already trying to sucker me into buying you stuff?”

“How about we make a deal?” She held her hands behind her back and leaned forward with a grin. “You buy me some clothes, you get to help me try them on in the dressing room?”

“… you mean-?”

“Fool around, yes,” she finished the thought for him.

After narrowing his eyes and furrowing his brow, but nonetheless taking in the girl’s figure one more time, Mike sighed. “Okay, yeah. But just this once! Don’t think you’re gonna get anything else out of me like that!” he said, pointing an accusatory finger at her.

“Heehee,” she giggled with a triumphant smirk. Holding up her own dainty fingers, she wiggled them as if manipulating strings. “Dance, puppet! Dance!”

“Knock it off, dork,” Mike said, rolling his eyes as he turned to continue on to the game store.

“Sure, I’ve got the knockers for it!” she said with a laugh, hoisting her bust up with both hands as she skipped playfully up to the keep pace with her friend.

“God,” Mike groaned at the terrible pun. “It’s gonna be a long walk to the store, isn’t it?”

“Need a ride, pardner?” she asked in a badly exaggerated (and quite girlish) imitation of John Wayne, holding up both hands like the hooves of a rearing horse.

“Ugh!” Mike grimaced. “You know, I’m going from being embarrassed for you to being embarrassed by you real quick.”

“Oh you’ll feel real embarrassed when I stomp all over that life gain deck you keep trying to build,” she teased. “Now you’ll get to add ‘by a girl’ to all the times I beat you at cards!”

“Don’t you have a meadow to go frolic in, or something?” he retorted.

“I say thee NEIGH!” she replied dramatically, pumping a fist into the air.

“Gah!” Mike cried, eliciting a cackle from his newly transformed friend as they continued down the crowded concourse.