“There is no wrong way to fantasize.”

Princess Celestia, ruler of Equestria, controller of the sun, headmare of the Canterlot School for Gifted Unicorns was endowed with tremendous responsibility, power, and wisdom. Preferring to rule as a taskmaster rather than as a tyrant and fight as a tactician rather than as a grunt, Celestia had taken centuries to hone her crafts in magic, government, and teaching. She had created and destroyed nations, won wars and slain great enemies, and been lucky enough to help train Equestria’s finest heroes. It was in cunning and foresight that she excelled. For Celestia knew better than anypony that she could not always count on her horn, wings, or physical strength. Despite her impressive size in each category as the largest Alicorn in the known world, she was a rather skinny pony with only one truly exceptional magical power. And despite everything – despite all the apocalyptic battles and generation-spanning plans and universe-saving students – she was still a mare, quirks and all.

It was a part of her that seemed hidden to many; buried underneath the pomp of royal processions and the otherworldly luminescence that seemed to radiate from her very presence. Not everypony got to see her small lapses: a practical joke at a tea party, licked lips during a baking contest, sarcastic remarks about boring public events, even gleeful prances at offers of stardom. She was a mare, and so she was, at times, curious.

Finding new interests was an essential component of Celestia’s life. Hundreds of years defined by responsibility alone proved draining, and endless repetition proved boring. Variety was the spice of life. Doubly so for a particularly long one. So, to her, there was no wrong way to fantasize, just so long as a pony enjoyed themselves and that enjoyment did not come to harm others.

That was Celestia’s very sentiment when she met her once-apprentice, turned Princess of Friendship, Twilight Sparkle, for tea one fair evening. Conversation about school administration, difficulties with sleep schedules, and inter-ministerial politics gave way to a healthy sprinkling of gossip. Of particular interest to the Sun Princess was one item regarding Twilight’s friend, Rainbow Dash:

“And you say she did this on purpose?”

“Yes, Princess. I invited my friends over for pancakes one day, and Rainbow started binging herself like she would never eat again. She said it wasn’t the first time she had done it either. She claimed these habits were just training for an ‘eating contest,’ and even went to all my other friends requesting bulk orders of calorie-packed food and weight gain supplements. I’m sure you can imagine what ended up happening.”

“A pony doing that for months? She must have grown truly massive!”

“She sure did. I mean, far be it from me to judge a pony who carries a little extra weight. And it did start that way at first. Nopony is immune to a bit of overindulgence and the, ahem, ‘results’ of indulgence: bigger belly, wider flanks. Even I have gotten that way while cramming for final exams…er, no offense.”

“Heh, none taken.”

“But Rainbow Dash ended the escapade trapped by her own fat and unable to move! If my friends and I hadn’t intervened, she may have just…”

“Kept going?”

“Exactly! The way she put it, the experience was enjoyable! I suppose if you took away the social stigma surrounding weight gain and added a belief that such excess was consequence-free, there would be no limit to how far a pony could go.”

“Consequence free?”

“Well yes, that is the other thing I wanted to talk to you about…”

Princess Celestia found this process of growth intriguing. Throughout all her lifetime, despite partaking in innumerable forms of sexual experimentation and debauched vices that would have ended a mortal mare, getting fat was not something she had ever done on purpose. Could it really be something that a pony can enjoy under the right conditions and with the right mindset? There were perhaps only a few other equines in the world who shared her impressive height and svelte proportions. What would it look like if her frame was covered by bouncing, jiggly fat? It was scandalous. Exciting, even. In that moment, curiosity met the cat. Though, she thought it best not to tip her hoof too early. Such an adventure would need to make some sort of sense beyond simple curiosity.

“…her first request was for a weight loss spell. And I’ve done it, Princess. I have perfected the first weight loss spell known to ponykind. And if I am being completely honest: I’m scared of it.”

Princess Celestia’s face twisted in surprise and confusion. Twilight was usually thrilled about new magical discoveries.

“How so, Twilight? Are new discoveries not meant to be shared and understood?

“The temptation of it, Princess. For all Equestrian history, it was known as a fact that excess weight could only be undone by diet and exercise. The dread of such hardship deterred most ponies from ever going too far. But with this power in hoof? Anypony who eats fattening treats and ends up getting fat because of it will have no reason to stop. They’ll just use the spell and be done with it. They may even go back eating the same fattening treats all over again. And then there are ponies like Rainbow Dash, who like the sensation of getting fat so much they will debilitate themselves before even thinking of getting thin again!”

It was a fair point. If ponies were no longer concerned about the concept of gaining weight, the social, political, and economic consequences of rampant gluttony and debilitating obesity would be unpredictable at best and calamitous at worst. What could be profitable for a baker might also be ruinous for a closeted fat fetishist no longer able to perform at their construction job. What could be stimulating for one partner in an intimate relationship might cause another revulsion and animosity. What would surely be a new stage in the development of magical knowledge could also be the first step towards bodily alteration magic as a common practice – super strength, gigantism, invincibility, mane restoration! The delicate harmony of Equestria would be at stake.

“Your concerns are well-founded, my faithful student, and I appreciate your candor. The consequences of making such a spell public are difficult to determine and carry undeniable risk…”

But what if a certain Alicorn wanted to try it? No reason to stop eating. Getting fatter and fatter (if only for a little while).

“…I suppose our options are clear. We can either destroy all evidence of the spell and rid our memories of its discovery.”

Celestia paused for a moment. Twilight’s eyes and ears drooped downwards at the thought of losing her hard work. Celestia knew her student wasn’t willing to do that. She had made her choice before the second option was even suggested.

“Or we can re-run the experiment.”

“What do you mean ‘re-run,’ Princess?”

“Your friend Rainbow Dash, knowingly or not, was the first pony in Equestrian history to gain excessive weight while believing it had no long-term effect on her health or wellbeing. If we are to determine whether her loss of self-control would be the norm or the exception, we need to try it again. Like any magical or scientific pursuit, we require a control subject put under similar conditions as the original incident. This case will be compared against a later, experimental group that can further test our hypothesis.”

“You are right, Princess! If we can prove that Rainbow Dash’s gain was somehow a fluke, we can see whether the average pony can handle the responsibility of magical weight loss. But what should be the test conditions? And how do we get a willing test subject?”

Celestia grinned and shifted her tone to the soft, wistful whine she used whenever a joke was obvious but somepony was yet to pick up on it.

“Oh. Well, we would need a fit pony with regular access to fattening foods, in a relatively isolated environment, who could plausibly gain an absurd amount of weight under any number of false pretenses, with nopony willing or able to stop them.”

Twilight dropped to her haunches and pointed an upturned hoof at the Alicorn who was beaming even more than usual.

“You can’t be serious.”

“Oh, I am. The final category for any ‘potential candidate’ is somepony who wants to try gaining and promises to stop themselves. You said your friend binged on…pancakes, was it?”

Princess Celestia punctuated her levity with a wink.

Author's Note

I truly believe that one can not contribute to the annals of mlp weight gain fiction without writing a story about Celestia. She's just perfect for it. I have made this story very direct, very tailored, and very considerate towards those who already have this fetish.

I may update this posting later with better story beats. For now, I just wanted to share.