Velvet looked over at her husband and son. The two mares were a mess. Their entire bodies shivered from exhaustion, barely keeping themselves seated on the mattress. Their sore legs threatened to buckle under the slightest strain and wobbled with each small step. Their bellies were swollen and sloshing, each thoroughly filled through all three holes.

Shining's fur had a more forgiving colour but Night's blue coat was now more akin to a work of modern art, stained white and soaked through and through with their combined juices. Their manes and tails were on another level still, now looking like glorified cumrags that would require hours of washing and cutting to get into any sort of acceptable shape again.

Not helping matters were their leaking holes, Shining's gaping ponut constantly dripping onto his tail while a seemingly constant stream of cum ran from Night's pussy and onto everything below him. Her son also had a rope of her spunk still trailing from his lips, he himself not even noticing or bothering to lick it up, while a similar one hung down from her husband's horn.

While clearly a success, Velvet's experiment had clearly spiraled out of control, and with their balls now finally drained perhaps it was a good time to call it a night.

"Alright, anypony else wanna try anything before I turn us back?"

"Nope."

"Me neither."

"No, you can get started. And don't let me forget to fetch some ice packs afterwards, me and dad are gonna be sore as Tartarus tomorrow."

"Why you two, wasn't it our pussies you were pounding, I mean we, I mean ugh, it's still confusing."

"I don't know about that, but it sure was my ponut that you two were double dicking earlier."

"Alright then, you three. We'll do it the opposite way now. Twilight, you and I lay on our backs and Night and Shining will ride us."

Contrary to earlier in the evening, Night Light now sunk his wife effortlessly up his marehood, and his son followed suit.

"Giddy up Twily, here I come!"

Once both mares had taken their seat the two Twilight's strained their necks, holding their horns together as Velvet cast her reverse spell. Again the room was filled with a spectacular light show, gray and violet mixing once again to the deafening hum of magic. The flash momentarily blinded everypony just as it had before as the spell reached its climax. Once done, the four ponies looked at their privates expectedly.

"Huh, what, why didn't it work? I'm still a mare."

"So am I. What's going on, Velvet?"

"That's strange, the spell should have reverted all changes. Twilight, what about you."

"Still a fifth leg."

"Hmm, the instructions didn't say anything about- oh no."

"Oh no? What's oh no?"

"Hold on a second, could you two get off again for a moment."

Shining and Night Light looked at each other worriedly before slipping off their partner's members. Velvet immediately rose to her hooves, walking over to the new mares, inspecting their bodies closely. Her horn flared to life and scanned over their privates, her face somehow growing paler even under her gray fur the further her spell traveled.

She looked up at her son, an apologetic smile forced on her face.

"Uhh, Shining, congratulations."

"Congratulations on what mom? What's going on?"

"Well, how do I say this? You're pregnant, it's quadruplets. At least."

"Wha-"

"And Night, let me check again. Yup, you'll be a mom too, with quintuplets, wait a second, nope, there's a sixth one."

"Pregnant? What? How? Velvet, what are you talking about?"

"Hmm, you see, remember when I mentioned that me and Twilight are in heat? Well, when we switched organs you two kinda received our fertile wombs, and we got your breeding instincts in return."

"Oh right. Usually I only have to worry about not getting a creampie from you, so I didn't even think about what would happen when I would give you one."

Twilight added in a shocked tone.

"And what does that mean?"

"Come on Night, do I really have to explain the birds and the bees to you?"

Velvet replied.

"Two horny stallions just spent the last couple hours pumping you two full with a few buckets worth of foal batter, all the while your ovaries were firing out one egg after the other. What this means is we need a bigger house. A much bigger one."

"P-pregnant, oh Celestia, we are pregnant. And what about the spell, why didn't it work?"

"It's a built-in fail safe, the spell doesn't work when it detects a pregnancy. Too dangerous otherwise. But look at the positives!"

"What could possibly be positive about that?"

"Well, you'll only have to stay like this for eleven months. Maybe a few longer if lactating blocks it too, who knows. Oh and you'll grow some massive milkers for me, so that's something."

Shining's blood had drained from his face completely by now. Desperately he pawed at his marehood, clenching his muscles to squirt the cum still inside back out.

"I can't have a foal mom, I'm not ready yet."

"Don't worry Shiny, it will be far more than one. And unless you want even more, you should stop clenching, that'll only push even more semen up into your womb. I can detect five in you by now, and there's still some eggs swimming around in our spunk. I do wonder why you two make for such perfect broodmares, that's definitely not from us."

"Oh buck, why did we do this? You talked us into this Velvet!"

"Hey now, it was you who wanted to try something new, Night Light! You got what you wanted, your first time being a mother."

"First time, more like only time."

"We'll see about that, my nostrils are full of pregnant mares right now and it's driving me wild. Who knows, I might put a few more buns in your oven once this batch is done. Oh and Twilight?"

"What is it, mom?"

"Be a dear and talk to Princess Celestia tomorrow. Your position as a librarian won't exactly cut it anymore."

"And what am I supposed to tell her? That I need a better paying job because I grew a dick and knocked my father and brother up with a whole herd worth of foals?"

"For example, yes."

"Ughh, alright. I'll write her a letter."

Author's Note

Apparently my workflow for these competitions is as follows:
Step 1: Read the prompt at the beginning of the month
Step 2: Come up with a concept
Step 3: procrastinate through the whole month
Step 4: Come up with a completely new concept right at the end
Step 5: Desperately type it out in a mad rush
Hope you enjoy it anyway!