Hissss
The sound of wetting her diaper made Twilight squeak in surprise, pausing her feeding.
Celestia gently removed Twilight from her teat and wiped her mouth as she cooed. "It's alright. Remember when you first peed your diapers? It's okay; you can't help if you're not big enough for the potty."
Twilight threw her new wings over her eyes, still feeling somewhat embarrassed about wetting herself in front of her mentor (and possibly doing the other thing).
The princess didn't mention it as she levitated the little filly into a playpen, placed her down with her Smarty Pants doll, and said. "Now, I think you should play with another foal to pass the time. I think you'll find this one to be very familiar."
Twilight was a bit confused about who she would be playing with, as it would be awkward for her to play with an actual baby. But then her questions were answered as Princess Celestia levitated a light blue coated baby alicorn, all padded up in a cute diaper with little faint blue stars and moons printed on the seat. As Celestia placed the filly next to Twilight, the baby alicorn gasped in surprise. She sat down next to Twilight as she shoved her hooves over her pampered crotch and cried. "T..T…Twilight!?"
"Trixie!?" The other regressed alicorn squeaked back.
The sun princess alicorn snickered. "Yes. Little Trixie here has been in the same boat as you for a month now," She ruffled Trixie's little man, "We've tried to keep things under wraps. Still, eventually, everypony will know Trixie is a cute baby alicorn now. She's part of the family."
"Stop it, auntie! Ya embewassin' me!" Trixie whined as she kicked about in her crinkling pampers.
Trixie then mumbled a curse under her breath as Twilight threw her hooves across her puffy chest. "Yeah, I remember when you tried to turn me into a baby like yesterday."
A few months ago
It was early in the morning with Trixie standing across from Twilight, glaring at each other. Twilight knew something was off with Trixie as she had been using advanced magic to bully Twilight's friends. She stood her ground, ready to take her own, wondering what kind of spell Trixie could use.
"Now for an age spell!" Trixie proclaimed as she aimed her magic right at Twilight! With a blast of red-colored magic, a cloud came over the two! Twilight coughed and gagged as she looked down at her body as it dispersed. She was pudgy and trim again, like when she had the Phoenix Flu. The unicorn then looked over to see that Trixie was struggling to get out from underneath her hat.
Trixie tossed the hat aside, revealing her small pudgy form with the alicorn amulet now gone. She had two wings protruding from her back. Twilight was somewhat surprised to see that Trixie was now a baby alicorn! Trixie didn't seem to notice that she was from a different tribe, yet she pointed at a hoof and shouted. "Ha, Trixie's spell may have misfired, but now ya a foal as well!"
"So what? Ya in diapees too!" Twilight babbled.
"Well, not really!" Trixie huffed as she tried to stand up. "Even as a foal, Trixie dun need diapees! Trixie was so gweat and powerful that she mastewed potty twaining before she was even a yeaw owd!"
"Nah uh, nopony can do tat!” Twilight retorted. "Now ya gonna pee pee all ova ta place!"
"Ya gonna do tat too!" Trixie snapped back.
Soon the two babies continued to argue, screaming and shouting at each other over who was more of a foal. And it went on and on until somepony shouted. "Girls! That's enough!"
The babies shut up rather quickly when they saw who had spoken: A very demanding Princess Celestia, who quickly levitated the two towards her. She then powdered their rumps and strapped up diapers over their flanks.
"Trixie sta-" Twilight squealed as a pacifier was placed into her mouth.
"-I don't care who started this nonsense!" Celestia snapped! "This has gone far enough! You both have been very naughty, messing about with forbidden magical artifacts! And look where it got you two, back into diapers."
Trixie giggled but squealed as she was levitated to face the princess, who snapped. "Now, Trixie, you have gone way too far trying to turn my pupil into a foal. As punishment, you will be her playmate and be a good foal to her while I search for a way to reverse your little age spell. I will permanently leave you as a foal if you don't behave. And we'll see how 'Great and Powerful' you are when filling you're pampered daily. So play nice."
"Yes, Mama!" Trixie cried, feeling somewhat surprised as the alicorn plopped both squirming babies into a carrier on either side of her. Twilight felt odd to be in the air a few feet off the ground.
"Lucky for you two, I anticipated Twilight might turn into a baby again. So I have a nursery already set up." Princess Celestia explained as she unfurled her wings and flew off towards Canterlot, leaving the ponies who'd been watching baffled and confused as to what just happened.