A number of young foals get stranded on a tropical island, like, forever. Maybe after the Equestrian apocalypse, or into another dimension or something. Yeah, they end up on a pristine Equestria w/ no signs of any past ponies anywhere. Escaped the windigoes?

So these foals engage in fantastic survival adventures, relentlessly making fun of the overly negative Lord of the Flies. Some get terribly sad b/c can’t find parents, but they hug and work through it together. And then one of the fillies gets old enough to go into heat. None of them got the talk. Isn’t long before they’re fucking like rabbits. Then some of their bellies start to grow, and none of them have any idea why.

The first one’s absolutely terrifying of course. She like... goes off somewhere alone to try and save them from whatever’s hurting her. Ends up coming back with a foal and they’re like WTF

“You know that place where a colt’s penis goes? She just started... coming out of there. She was what was in my belly! I—I don’t know how! I just... she started coming out, and I just had to keep pushing and pushing.”

Starts happening to a second, and everypony is paying attention now. She gives birth, and... the first filly very leerily admits there was something else that happened to her, after the birth. She tries to describe what’s up with the cord coming out of the second filly’s snatch. Says it’s not bad, just something else that has to come out.

Then everypony freaks out at how gross it is. The first is all apologetic and says to the second that you can eat it, and that’s what she did. The second’s horrified, but intrigued, as the first says she was just so tired and hungry, and just kind of started worrying at it. Second eats it with semi-supervision, and first says the cord just falls out on its own. Doesn’t know why it’s stuck to the foal’s belly button.

The remaining pregnant fillies are completely freaked out by what’s happening to them, but at least not worried about something <i>bad</i> in there. Lots of speculation that maybe if your mommy’s not around long enough, you just start turning into a mommy yourself. Somepony makes enough of a connection to think that <i>they</i> were in their mommy’s bellies once, but other than that it’s ignorance all around.

Though a few fillies are worried about the colts “messing up” their foals or something, especially after the first miscarriage. Others are arguing that it’s good for the foals, and that the boys are doing it to them to make the foaling easier. Like exercise and such. A few really bummed out that they have to be moms now, but most find their little wild foals delightful. Because... that’s kind of what happens in real life... sans pony and wild lands and such.

Then they’re getting to be adults, and still having sex w/out restraint. The fillies are shocked when one starts getting pregnant even though she already <i>has</i> a foal. “Is that how twins work?” “I don’t know!” A lot of the (new) foals kind of... die in various comical ways. Stuff eats them, they eat a berry that makes them explode etc. So the population is <i>sort</i> of stable.

Of course long before their foals go into heat, they start observing their parents having sex, and trying it out, with full encouragement from parents of course. Disappointment when the prepubescent colts can’t ejaculate, so the older colts/stallions are more popular with the younger fillies b/c novelty. And the older females tend to go for the colts, because it makes them feel nurturey.

Colts start getting old enough to ejaculate, the ones who survived that long at least. Older ones start treating it like a rite of passage or something. You’re in the big boy club now. Also makes them popular with fillies their age, who’re starting to be interested in them again.

That’s actually years before a filly’s first heat, so she’s getting cummed inside for quite a while before going into estrus. But then she starts getting a bigger belly, and everypony’s like, “Woah, our <i>foals</i> can have foals!” She gets treated kind of preciously, by curious ponies wondering if she’s going through what they went through. Eventually has her foal, confirming that she was indeed getting pregnant, just like the first generation.

Another bunch of fillies start coming forward saying their bellies are getting bigger. They don’t know why though, because estrus was 7-9 months ago, and they really have no way to connect the two events. Some of the nursing fillies are gratified/disappointed that they don’t get completely obsessed with being mounted anymore, when it happens to the other fillies. FTMP, estrus is seen as a positive thing, some magical season where everypony’s in the mood, and everypony can get all the sex they want.

A few foals resemble their father of course, but these ponies have been so promiscuous that all they can tell is that the foal’s fur or hair is kinda like that guy’s over there. No real idea about genetics. Adults are getting really good at surviving, fending off storms, raining on crops, growing crops. The oldest generation remembers stuff like baking and cookies, but recreating that’s a difficult task. Some start trying to find ways to record what they know, some sad that they went so long and forgot so much before thinking to use their memories.

Somepony figures out how to mill flour though, and from there and cows it’s only a matter of time before another pony comes up with a delicious batch of cookies. Ofc what they remember is all the sweets and enjoyable things, and the regular food they eat’s just... tribal stuff like grass and flowers.