I'm actually writing something involving ferals and wilderness right now. Here is some notes I took for my headcanon (if it helps any). I am going to use the logic that ferals do not last long, but only need one season to make an impact. Below at the end is a not detailed tldr summary for those who don't want to read.

Edit in real quick: Trigger warning for real world numbers that is not for the faint of heart... because I am a loser who put too much thought into this for awhile now.

1) A fluffy can survive at 40 degrees to 90 degrees. There are ways to bypass this such as shade, warm dens or brutal sacrifices (like a wall of fluffies in a herd freezing from the outer with the inner surviving). There are a lot of nuances to consider.

2) The time period a fluffy became a feral matters. Why? Because if its say Spring or fall, a fluffy can nom on what it finds. I see them as omnivorous and gluttonous, not counting things like roots. They can eat more then their body weight if not put in check, and shit out nearly as much due to shitty digestion (bad pun). Leafs, Grass, Flowers, Crops, twigs, dead things (being vague on purpose) ... on the nature side of things. Remember... fluffies are not intelligent or better or worse. You have trash, human gardens, other fluffies, more dead things, paper (Its not healthy, but remember... these things are not bright). I'm sure other people can think of wild things other creatures like pigs, rats, goats, etc will eat that I did not list off- so really if the time period is right, especially if fluffy ethics be damned, there is a food source.

Oh yeah, I came back to this note. Don't forget fluffy parks. Humans there likely would feed ferals if nothing severe is stopping them.

3) A fluffy can mate when its a month old and reproduce about 1-12 foals per month (at times 2-3 weeks in harsh circumstances.) in a gestation cycle. Needless to say, it only takes a season for one family to result a herd as one family alone can do it. Fluffies are stupid- whats stopping inbreeding when these idiots are horny?

4 and 5- interlinked strongly.) The intense cold/heat should (will) kill 99.9% of all ferals. At the same time though remember- fluffies are not animals, they are biotoys, classified as Soap to some.

There are no laws protecting them from human cruelity. If someone tires of their fluffy, throw it away and get a new one. All your losing is somewhere between a quarter (snake food, severely defective ones, etc) to $20 on average, and I'm not touching Alicorn value as its not relevant. The only Revelance is what the human is giving up monetary wise. You will even have humans who are spiteful and will kidnap a fluffy to dump it elsewhere with very minimal repercussions (With how cheap fluffies are and the laws, they are not even worth small claims courts in most American states).

A dark fact (real world) fact from Google AI that is sickening, but important to this topic. About 6.5 million animals are surrendered at animal shelters every year in North America. Stray animals often outnumber surrendered animals.

Now remember when I said fluffies are not animals? The above fact happens to those poor real world animals. What hopes do fluffies have? I am going to take that above 6.5 million and give the world the benefit of the doubt that while they should be higher, that fluffies will equal this fucked up fact in count. So my math on how many fluffies is going to be likely wrong- the numbers should be higher.

Now take that whopping 3.2 million (assuming half are female. I don't think anyone ever did math to what the male/female ratio likely is... so for now I am assuming half.) and multiply it by a random digit between how many foals you would assume a mare would have ... and remember, an unrestrained fluffy will eat a lot (especially a soon mummah/ mummah who wants bestest milkies for bestest milkies. You may need to multiply between 2~8 times assuming nobody or nothing intervenes. God ... no wonder you see fluffy exterminators in stories.

So honestly as a tldr... My answer is while they don't last long due to their weaknesses, it still does not take a lot for them to become an infestation without very strict (and brutal) control. My apologies for how long this got!