It is my understanding that when when Bjornaer magi undergo Final Twilight, if they had been initiated in the inner mysteries, then they become a Great Beast.
Also, these Great Beasts are effectively unaging and nearly immortal.
So what limits their population, so that every wild place in the world is not eventually occupied by at least one Great Beast?
Basically, in your saga, what kills/ends Great Beasts?
They don't reproduce sexually.
They are at least partially maintained by mystical energy.
They haven't had time yet.
Eventually they go live in or create magic auras, which regionise.
The requirements are sufficiently difficult that there simply aren't that many Bjornaer who reached that stage successfully. First, they have to be Bjornaer. In the entire history of the Order, that is a population of several thousand. Second, they have to have been initiated into the Mystery of the Inner Heartbeast. A magus won't find out that this is even possible until they have raised their House Bjornaer Lore to 5. Before that, they know that Great Beasts do exist, and arise (rarely) from some mechanism that they don't understand, but they don't realize that is possible for themselves with time and effort. So less than a thousand of all Bjornaer - ever - learn enough to consider that path. Some of those won't pursue it, and some of those who try will fail. Of those who learn the Mystery necessary, only those handful who reach Final Twilight will transform into Great Beasts. Death of old age or mischance won't do it.
So the number of Great Beasts is fairly small.
And their ultimate Might is based on their score in Heartbeast, a skill that isn't terribly important to get to high levels until you learn Inner Heartbeast, and one that has few, if any, books to learn from. Likewise House Bjornaer Lore---the tiny number of written works would be carefully secured and be authored exclusively in Gothic. Advancement would be slow and require Stories - the human-friendly Bjornaer of Vienna won't be able to curl up in a comfy chair in a library to reach this outcome. A Might higher than 30 is going to require significant dedication, and a weak Beast is at risk from other monsters and possibly Vis-hunting magi.
Nice break down of chances. With these we can build a sort of Fermi equation to estimate the number of these Beasts!
The Order, through all history, is probably still under ten thousand magi (I'd say it should be around seven thousands right now), so Bjornaer would have a population of several hundreds, not thousands.
If I had to wild guess that number of Great Beasts I'd say 25.
The total population of the Order of Hermes through its history is several thousand. The entire history of the Order is only about three or four Magi lifetimes. House Bjornaer would be several hundred total members over the lifetime of the house.
If we reduce your numbers by an order of magnitude then it would give us a fairly good idea. There might be 100 Bjornaer total who are capable of becoming a Great Beast. The actual number who successfully pull it off would be far lower.
On the one hand, player character Bjornaer nearly all want to follow the mystery of the Inner Heartbeast (although how many have a saga last long enough that they can?) Also, NPC Bjornaer are more likely to have an Inner Heartbeast because the storyguide wanted a cool character with funky special abilities. So, based on what actually turns up in people's sagas the proportion of Bjornaer with Inner Heartbeasts may seem higher.
On the other hand, Timothy's point about them forming magic auras that turn into regiones is a good one. I would go one step further and say that some Great Beasts find a way into an Animal area of the realm of Magic and so largely disappear from our world.
In my Rhine gorge saga, Urgen ex Bjornaer has demonstrated his mastery of the Theriomorph path as part of a demonstration of strength.
He also advised the party that "meat is meat" and "do not waste meat" as part of the same conversation on the wizard's war he was engaging in.
I realised that there are currently not many Great Beasts, though exactly how many is up for debate. My issue was that so long as House Bjornaer continued, there would be a supply of new Great Beasts, so eventually the Mythic Europe wilderness would become quite crowded.
I didn't realise that Great Beasts could not reproduce. I don't recall that written anywhere.
Several Great Beasts are probably unique, and there is (currently) nothing they can mate with (until another Great Beast like themselves occurs). But I was under the impression that a Gryphon Great Beast would probably be able to mate with a "normal" griffon.
The disappearing regionne is a neat idea.
In my saga - Great Beasts essentially reside in the Magic Realm. No matter how many there would be, they won't occupy the wild places of the world. They might stroll through Magical forests and such occasionally, especially if there is a cultist there looking for them, but that's about it.
Whilst there will be a trickle of new ones, old ones are also going to die occasionally, and the death rate is probably going to go up if overcrowding starts to become an issue.
I also don't think that the offspring of a Great Beast and something else (even another Great Beast) would itself be a Great Beast - having formerly been a Bjornaer is an inherent part of the definition to me. They'd just be an animal (and if their mate is a mundane animal, probably just a mundane animal).
This goes back to the original point of the thread
What, in your saga, are the leading causes of death of Great Beasts?
I am looking for inspiration.
Flambeaux and Tytalii looking to test themselves against the most dangerous game, followed by pack hunting Tremere who feel threatened.
In practice in my sagas it's never come up. However, I'd agree that vis/trophy hunting magi are definitely going to be up there - it's unlikely that most of them will know that the animal they're killing is a former magus (and whilst if they do know it will stop some magi, former magi are probably not protected by the Code).
This may very well make said magi unpopular with Bjornaer, of course - if they find out.
Depending on the animal, you may occasionally get them killed by mundanes as well, especially if said Great Beast is protecting a bit of wilderness that a population wants to develop.
Because House Bjornaer has Inner Mysteries that can be taught by these Great Beasts, they could claim that a Magus killing such a creature is depriving them of magical power. Few Magi will want to risk taking on several angry Bjornaer...
Yes, in principle. That would mean exposing some of the secrets of their mysteries to the Tribunal, however - remember that you need a House Bjornaer Lore of 5 to work it out on your own, so it's far from common knowledge. I think "being killed by an angry bear in justly declared wizard's war" is a more likely approach, and still a significant deterrent - if you know what you're doing.
Almost all Great Beasts would have taken longevity rituals long before their transformation so infertility would be an essential aspect of their being once they transform. Obviously this might change if an LR that preserves fertility is discovered and spread within the Order and there may be a few who have sought other means of life extension that don't make one infertile.
I do not necessarily agree that the infertility is an 'essential aspect' of their nature. Its a fine interpretation that they'll be infertile due to longevity rituals taken beforehand, but I don't think it's part of their essential nature any more than getting stabbed in the eye makes them 'One Eyed' Essentially - the latter can be healed via magic.
I'd argue infertile- yes, Essentially infertile, no.
Their state at transformation into a being with magical might is what they are in an essential way. It's a matter of becoming an unchanging magical creature not that it was necessarily an essential trait before they changed but that their state when they changed is what they have trouble deviating from and that state is infertile.
Just recalling there is a couple of mentions in canon of Familiars producing offspring (lineages of magical cats, the magical animal companion of the beggar king of Paris).
And Familiars share in the Longevity Ritual effects, don't they ?
Familiars are not directly affected by a longevity ritual as far as I can tell, only indirectly.
Familiars age along with their magus, and won't die of old age until the magus does.
So if the magus ages slower or quicker than they would naturally do, then so will the familiar, regardless of why the magus ages slower or quicker.