I've been thinking about this for a while as I've always wondered if bat ponies are a fourth member of the equine race, or are more like kirin who, while sharing multiple features, are not ponies. After doing a little research I discovered that bat young are called pups and wondered if that would be a more appropriate term than foal on the basis that they aren't a race of ponies.

What are your thoughts?

7043887
In my opinion, I believe that Bat Ponies should be accepted as the fourth pony race, since I believe they are absolute fascinating and very beautiful in their own right. Besides, we haven't really seen them that much on the show, other than that one clip when Luna came to Ponyville on Nightmare Night in the episode of Luna Eclipse. As for the term to call the bat pony foals...if I remember correctly I red a few stories on bat ponies, the foals especially, and they were referred to as fledglings.

Interesting thought... but since we tend to use 'batpony' and 'thestral' interchangeably, I consider them another subspecies/subrace of ponies. They are still 'ponies' but they've got a little something extra in them. That and they're fully compatible with other pony races without any magical higgery jiggery.

So I'd just still call them colts, fillies, foals, etc. That's just me though. I could believe 'pups,' too

7043903
"Pups" is legit. The term is used to describe young Flying Foxes.

Calling Thestral/Bat Pony young "pups" could be a cultural phenomenon.

7043887
Seeing as there's really no established canon on batponies, you can really do whatever you want, as long as the rules you create about them are kept consistent. Personally, I've always seen them as a sort of subspecies of pegasi that evolved differently. Some people go the blood sucking route, essentially turning them into vampires while others go the fruit bat route, which I think fits much more in line with the overall feel of FiM. Really, the beautiful thing about it all is that you can do whatever you want. Just do it well and people will enjoy it.

To answer you're original question though, the idea of calling young batponies pups instead of foals is actually a really nice change that I dont think I've seen anyone do yet. Stories that have these little tweaks, I find, are often more interesting and original, having more of a genuine feel to them. For this specifically, I think you would need to decide if batponies are more pony or more bat, or whether or not you really think that matters.

In my eye's batponies AKA Lunar pegasus AKA thestral's, are in part Luna's children of the night as told by this song.

SO they where regular pegasus at first along with the other orphaned and abused unicorn and earthpony foals; but when Luna took them under her wings she cast a maga spell on them as a group and made them into her children of the night, and thus we have the Batponies, Saberhorns and Nightwalkers.
Note to self my sona's a nightwalker

7043902
We've also seen them in cutie remark in Nightmare Moon universe along with RD (who wasn't actually shown as a bat pony)
But if you considered the idw comics as cannon bat ponies do exist especially when they fought against the nightmare creatures and were present at the all creatures convention

Other than that I pretty much see them as a rare subspecies of pegasi

7045425
Oh Right .

7045425
content://media/external/file/114209

If we're going off of standard bat pony fanon (tufted ears, fangs, leathery wings), there isn't much separating them from the other subspecies. Certainly not enough to not qualify them as ponies. To do that would require inventing features and biological traits that are outside what is regularly done. As an example, there's the old 2012 bat pony fanon that they're really longmas.

7043887
The truth is that the Kirin are originally from Chinese Mythology, where they're called "Qilin"(pronounced Chee-Lynn), in Japan & Korea they are called "Kirin" or "Girin"(though "Kirin" is the Modern Japanese Term & "Girin" is the Modern Korean term for Giraffe), in Thailand they are called "Gilin", in Vietnam they are called "Ky-Lan"(pronounced Key-Lynn), but despite being referred to as the "Chinese Unicorn", the Kirin(Qilin, etc.) is actually a chimerical hybrid of several different creatures, though for the most part it is pretty much some kind of "Dragon-Deer", mainly due to the Dragon-like Scales & association with Fire, and the Deer-like Body & Antlers, some even say that it is believed to somehow be a relative of the Sika Deer.

But the Bat-Ponies are basically Pegasi with more Bat-like attributes instead of Bird-like attributes.