The Hogwarts Thestrals, in Detail
After writing down a few thoughts about the missing Hogwarts magical menagerie last time, it still left a few things on the topic completely untouched - namely, the thestrals. So let's take a curiously detailed look at the thestrals and the effects of their addition to Hogwarts grounds.
Just so that we're all on the same page, a little summary on the basics: thestrals look like reptilian and skeletal horses with wings and blank white eyes, although are invisible to anyone who hasn't seen someone die. They are described as carnivores on Pottermore, but it's unknown whether they're scavengers or predators. Additionally, they are larger than horses and have far more stamina and speed than a horse - the last we know because Harry & co rode thestrals from Hogwarts all the way to London without stopping, and in a few hours at that.
What's this then, you might ask. What exactly do the Hogwarts stagecoaches have to do with thestrals, beyond the fact that thestrals pull them? They can tell us quite a bit as it turns out, as that particular word is not just a random synonym for 'coach' or 'horseless carriage'. This word was chosen in specific over all the other words that could have been used, and it has Implications.
A stagecoach is a horse-drawn four-wheeled public transit coach, and they were the fastest way to travel around prior to invention (and widespread availability) of railways. They were first developed sometime 13th century, although didn't become common until early 17th century, at which point it was Britain that spearheaded the revolution. Over the centuries there were different models of them of course, but we do actually know roughly what era of model Hogwarts used.
Rattling and swaying, the carriages moved in convoy up the road.
(OotP, the convoy on its way up the carriage road from Hogsmeade Station to Hogwarts)
See those word used? 'Swaying', in specific? In all early models of stagecoaches, the motion was a jarring up and down bouncing. The swaying and rocking only became a thing in 1867 models and later, not too long before railways replaced them as the major choice for transport. This, as it turns out, is relevant in at least one nitpicky way not strictly related to thestrals, and that is the number of passengers.
An early model stagecoach had two seats inside the carriage, one front and one back, and each could seat three people. The swaying models though, expanded that with a third seat for up to nine passengers inside the carriage. Later models added even more, some rather imaginative models even expanded up to 20(!!) passengers total, but I think it's a rather safe option to go with the standard nine passengers. Furthermore...
[...] followed the rest of the school along the platform and out onto a rough mud track, where at least a hundred stagecoaches awaited the remaining students.
(PoA, Harry entering the stagecoaches for the first time)
Here stood the hundred or so horseless stagecoaches that always took the students above first year up to the castle.
(OotP, taking the stagecoaches up to Hogwarts)
As you can see, it's entirely definitive that there are just over one hundred stagecoaches sent to Hogsmeade Station every year. Perhaps 110, to pick a reasonable number that might just be discernible at a glance from an even one hundred, if you're good at estimating the number of things. That totals 990 possible passengers, perhaps even more.
Even should you find that hard to believe, the books themselves confirm at minimum six passengers for every coach, as in OotP we see Harry, Ron, Hermione, Ginny, and Luna all enter one stagecoach, while Draco, Crabbe, Goyle, Pansy, and at bare minimum one other Slytherin student entered another one. So should you assume that the Hogwarts stagecoaches are small like the early models, that's still space for some 660 students that they send down there.
Aside from all the other indications in the books that Hogwarts has some 800 students while Harry attends, this surely must be the final nail in the coffin of that tired old fan theory that Hogwarts has only forty students per year, or 280 total. I repeat myself here, but even if they for some reason acquired smaller old model coaches with new ('new') anti-bouncing technology, that's still space for more than 600 students. Since first years don't even use the stagecoaches they'd send space for almost 700 students when there's only some 240 passengers. Who does that?
No, it's rather certain that there are roughly 800 students at Hogwarts as stated elsewhere in the books and confirmed in interviews, the stagecoaches are the new models and thus carry nine passengers each, and the convoy of a hundred plus stagecoaches sent to Hogsmeade Station has space for some 990 students. This has space for all the students and also conveniently allows many of the stagecoaches to leave when only partially full, to eliminate long waiting times at the station while students desperately try sort themselves into coaches while still staying with all their friends...
The stagecoaches turned out to be a rather useful diversion already, wouldn't you say? But there's more.
To find out what kind of effects the addition of thestrals might have had around Hogwarts, one should be aware of the size of the Hogwarts herd. Now, taking a look at muggle stagecoaches, they were drawn by teams of four or six horses. But we can't draw exact parallels here, as we're specifically told thestrals are bigger than horses (in the Fantastic Beasts and Where to Find Them minibook), and they are also stronger so not as many would be required to pull a single stagecoach.
But what do the books say on the subject?
The coaches were no longer horseless. There were creatures standing between the carriage shafts [...]
(OotP, Harry's first time seeing thestrals)
“At the — there, between the shafts! Harnessed to the coach! It’s right there in front —”
(OotP, a bit later in the same scene)
Unwillingly, because he had half hoped they would have vanished, he turned his eyes instead upon the strange, skeletal creatures standing quietly in the chill night air, their blank white eyes gleaming.
(OotP, when getting off the carriages)
The evidence is..fairly inconclusive. The first and third quote seem to indicate that there's two or more per coach, but in the second it looks like there's only one. We're just going to have to be content with an estimate of 1 or 2 per stagecoach. I'd personally say two per coach, but the text doesn't back it up any more than it backs up one per coach.
The number does create a bit of a problem though, as there's roughly 110 stagecoaches. It then follows that the size of the Hogwarts thestral herd must be at minimum 110 or 220 adults, plus any young or elderly thestrals that can't pull carriages.
Where on earth are they stashing all of those?! Well...
A great, reptilian winged horse, just like the ones pulling the Hogwarts carriages, with leathery black wings spread wide like a pterodactyl’s, rose up out of the trees like a grotesque, giant bird. It soared in a great circle and then plunged once more into the trees.
(OotP, Harry looking out a dorm window at night)
The few individuals Hagrid introduced the CoMC class to might have been brought further away from the herd, but thestrals wandering about in the Forest at night does seem to point to them roaming the Forest freely when not working. Oh dear. So Hagrid really just...stuffed them in the Forest and called it a day. Figures.
We're missing key pieces of information here of course, such as how much a thestral eats at a time, how often they have to eat, how much of their meat is provided by Hogwarts instead of hunted/foraged for by the thestrals themselves, and how much living space each individual requires - they're herd animals but even herd animals have requirements for personal space. Regardless though, the addition of a hundreds-strong herd of carnivores to the Forest must have really done a number on its ecological balance. You can't just suddenly add that many large living things in there and expect it to work out! The prey animal populations will collapse!
And should you doubt it, the addition of the thestral herd to the Forest is a recent thing:
“Hogwarts has got a whole herd of ’em in here. […] ’Course, this lot don’ get a lot o’ work, it’s mainly jus’ pullin’ the school carriages unless Dumbledore’s takin’ a long journey an’ don’ want ter Apparate […] Yeah, so, we started off with a male an’ five females. This one,” he patted the first horse to have appeared, “name o’ Tenebrus, he’s my special favorite, firs’ one born here in the forest —”
(OotP, Hagrid during the class he introduces thestrals in)
As we can see here, the Hogwarts thestral herd was all bred and trained by Hagrid, which means the first individuals were acquired in 1943 earliest - the year Hagrid was expelled and after which he was hired on as a gamekeeper (or hopefully, gamekeeper trainee). Which is to say, they've been around at most fifty years, and a considerable time before their actual work pulling the stagecoaches they had to have been doing something else, since there wasn't enough to transport all the students. You don't go from six thestrals to hundreds in a heartbeat, it takes at least a decade or two to build up a herd of that kind of size.
It could of course be that there were wild thestrals in the Forest before Hagrid's breeding program (in fact, extra-book canon is firm that there were wild thestrals around Hogwarts before then, and even Pottermore says they're native to British Isles), those could have boosted the semi-tame herd's numbers quite fast and somewhat mitigated the ecological damage the addition of a herd that size would cause. It's still no laughing matter.
Aside from Hagrid's careless attitude towards introducing foreign species and very large herds of carnivores to the Forbidden Forest and causing potential ecological disasters, which I'm sure we're all already familiar with (see also: acromantulas), the thestrals do raise some interesting questions.
Namely, how did the older students get from Hogsmeade Station to Hogwarts before the thestrals? The first years' boats mean that that part of the tradition has to have changed too if no transport was available for the others, as the first years would arrive at the castle a fairly considerable time before everyone else if the older years had to walk (see Harry and Tonks's trek in HBP). This leads me to believe that the stagecoaches themselves are not a new addition, even though the thestrals pulling them are. That would then necessitate the use of non-magical horses or other winged horses than thestrals, before they took over.
And lo and behold, the FB minibook provides us with aethonans - a chestnut variety of winged horse that it reports was popular in Britain and Ireland. There is absolutely no reason to think this theory has to be true, but I think a Hogwarts aethonan herd that far predates the thestral herd would explain quite a lot. It doesn't explain what happened to the aethonans of course, since we never see any and nor is their existence ever hinted at, but it would allow the students a way to travel all the way from Hogsmeade Station to the castle before the thestrals, and it would explain why the stagecoaches are a mid 19th century model if the thestrals were only added mid 20th century or later.
I suppose the aethonans could have been sold when the thestrals took their job, or the herd dwindled over the years naturally and we simply don't see the remaining few individuals...just like we don't see all the dozens of kneazles and puffskeins and nifflers and all the other creatures that Hogwarts should by all sense keep for its Care of Magical Creatures class. It would mean, though, since aethonans are rather more unlikely to fare well if left to themselves in a forest full of magical predators than thestrals are, that there's likely the remains of a very large paddock for grazing (winged) horses somewhere close by to Hogwarts, large enough for some 200+ individuals. A bit of research tells me that for a horse herd of that size it should be at least 201 acres. We already know that winged horses tend to be larger than non-magical ones though, so it'd probably be even larger than that. Hogwarts seems to have many mysteries we fans have yet to uncover.
Fun fact: McGonagall and Sprout both went to Hogwarts early enough that they most likely saw these hypothetical aethonans! Even Arthur and Molly Weasley (and therefore others of comparable age, like Bellatrix Lestrange and Rita Skeeter) might be old enough to have seen them before they were replaced with thestrals. The possibilities!