@Background Pony #4BB9
 

 
Which is true, but I think it’s important to draw a distinction between a hybrid created by two different species reproducing and a chimeric being that happens to have the body parts of more than one creature. Even in the original mythology, centaurs were not the offspring of humans and horses, and griffons weren’t created by the mating of lions with eagles. They have the characteristics of some or many other creatures, but they’re not generally treated as being created by those species mating – they just have body parts that look like them.
 
MLP certainly has had true hybrids on the show – mules come to mind – but I don’t think that it also having chimeric species within it is the source of the fandom’s use of hybrid offsprings and next gens.

@Frustration in Excelsis  
If you think about it, the show makes pretty heavy use of supposedly hybrid lifeforms: Cockatrice, Minotaurs, Manticores, Bugbears, Centaurs (maybe), Chimera sisters, Draconequus, Hippogriffs.  
And in addition to that we have a wide variety of lifeforms more or less derived from the combined attributes of real life animals and humans.  
The fandoms of shows are known to take the shows topics to new levels so it makes sense that we would create crazy offspring with all that stuff but it might be weird if the show actually did it considering how weird of an idea the changelings already are.  
They could be the limit.

@mana the immortal
 
That’s actually a fascinating subject, because it’s not that uncommon for different species to be able to interbreed – wolves and coyotes, for instance, do so in the wild with some regularity, and the hybrids are fertile enough to have nearly supplanted both parent species in some parts of the Northeast Atlantic seaboard. It’s almost always between species in the same genus, but even then there are exceptions – domestic cats will fairly readily mate with leopard cats to produce fertile young, to the point that there’s an entire cat breed that was developed by breeding the two animals together. There are even dromedary-llama hybrids around, although they require artificial insemination due to size differences (dromedaries are quite a lot bigger than llamas).
 
_{(All links go to Wikipedia)}
 
… to get back to the point, I don’t think it’s necessarily that far-fetched to assume different species can produce offspring. That being said, I would agree that beings as obviously different as equines and arthropod-equine chimeras are… well, unlikely to successfully produce young, at least outside of a laboratory setting. If I were to make my own AU or next gen (doubtful, but let’s say), I would probably not make them able to reproduce without shapeshifting, which would likely result in the child being of whatever species the changeling shapeshifted into. But people are still perfectly free to decide whatever they want for their own works, of course.
 
On a more meta note, personal opinions aside, I don’t find it too surprising that fanon offspring for horses and bug horses have been showing up. After all, draconies have been a thing since… well, since Sparity has been a thing, really. Changeponies are a much less extreme form of hybrids, and with draconies as common and generally accepted as they are, the addition of changeponies isn’t comparatively a very big jump to make.
 
@Background Pony #4BB9
 

 
True, but they’re likely not in the show. At least, there aren’t any humans to be the minotaurs’ progenitors.

@mana the immortal  
We also have Minotaurs and Cockatrice which are hybrids according to old myths. It also depends on the interpretation of the myth since there are always multiple versions and interpretations of it. In some of them the hippogriff is the offspring of a griffin and a mare.  
They could be hybrids or not but they at least seem related to other species in some way.

@Background Pony #4BB9  
Hippogriffs and kirin aren’t related to actual equines according to old myths. Heck, kirin are closely related to dragons.

@Frustration in Excelsis  
Well, the special thing about the changelings is how they transform into other things and then steal love.  
It stands to reason that there is something that has prevented them from being able to bear offspring with said things in the past in case they actually make love with them like the way their powers work or the whole stealing love thing might take the very magic away that is needed to breed.  
Or they have some conversion mechanism that turns other offspring into changelings themselves.
 
The new transformed non-parasitic changelings who don’t steal love might be able to bear offspring with other creatures the same way they can. We have hippigriffs and kirin so it’s not that far off actually.

@Frustration in Excelsis  
Well, given how they’re definitely distinct species, and not just different “breeds” I find it hard to believe. At best they’d have offspring that isn’t capable of further breeding imho.

@mana the immortal
 
Perhaps not and perhaps yes. It doesn’t seem like the kind of thing the show is likely to even tangentially approach.
 
In the end, I suppose it boils down to how much one headcanons magic to be able to account for biological incompatibility.

Would changelings even be able to have hybridized offspring?