I disagree with the livestock-> citizen progression for "civilized" races, as they (currently) don't accept slavery.
Slavery covers a different aspect, I mean moreso out of intelligent tamable creatures, slavery in the game as current & theoretically is the with the proper ethic settings, the resettlement of people from conquered sites and theoretically subjugating local animal-men through force & making them join your civ settlement rather than 'taming'
Gremlin taming is not slavery, neither is animal men elven migrations, but forcefully removing X tribe of animalmen in its entirety for a region and pushing them inside the nearest dark fortress/civ building would be.
I think it would fit goblin society to have trolls and other "wild" (semi) sapients captured in raids and kept as livestock for semi sapients and adopted members (after proper indoctrination and quite possibly indefinite slavery) for full sapients. Worthy slaves and semi intelligent "livestock" could be promoted to citizens according to various rules (that need not be consistent either within or between fortresses).
It can be noted that goblin trolls have names when invading and they can come as both monster invaders and as soldiers. In the first case they still have clothes, while in the second case they are also armed (I'm not sure, but they may have a last name as well). A related bug is that beak dogs can come as "soldiers" as well, and it's been reported they've been riding beak dogs...
I agree on the raid/slavery mechanic (see previous point about removing X tribe or X creatures from regions) and only partially agree on rules, given that having a ability to grasp & speak should be enough to become a citizen, and the rest remain livestock slaves (there is no intermediary) trolls do not speak, are not fully intelligent but can grasp so fufill part of these rules.
Both of those instances are not indicative of intelligence, the [EVIL_ANIMALS] tag natually flags up trolls as evil semi-intelligents for use as non-declared war animals, same for (no tags like war animal or tamable) beak dogs also being used. Those names are acquired in the same way as animals getting kills given that nicknames (gained via getting kills) and actual names are separate.
> In GM editor, trolls have no true name & will only aquire surnames - AKA. Terry Wogan, simply becomes 'Wogan' Tame Troll like a adopted animal, rather than 'stray tame troll'
For (dwarven) fortresses wild sapients ought to be able to visit the fortress as other visitors in the form of "tribals" or some similar status, but otherwise behave as other visitors (i.e. offer their martial prowess for hire, or perform, or just come to gawk at civilization [and get drunk] etc. When magic comes along they might also offer shamanistic clerical services and magical research). Once citizens they'd be normal citizens.
Well that'd would mean detracting from the mindless wandering animal people naturally milling about by giving them too much player interaction and removing whatever aspect of antagonism they have (not to mention visitor bloat, there are more types of certain animalmen than others, months and months of skill-less fly men)
Alternatively you could offer a identity and a mini-culture to said animalmen so their regional presences are more distinct, as to pinpoint where a elven migration comes from rather than drawing on obscure references. There used to be behaviour like this prior to 42. in a earlier version with savage tropical grassland Hyena men poaching your livestock to take back to tents usually constructed out of above ground spider silk.
That i feel would give them a bit more depth and properly help with your tavern recruitment idea, so it comes from small communities (though that'd be opening a jar of worms to certain below ground citizens) that can be displaced (see first point's slavery idea) rather than just drawing from the pool of animalmen
Semi sapients are trickier, but they certainly might show up as mercenaries, and might be capable of reasonably simple labor (hauling, wood cutting [after all, if they can cut enemies they ought to be able to cut trees], threshing,... Semi sapient visitors would have a name when arriving as visitors, but would just be unnamed savages when visiting the embark as "animals" (although it would be cool if a savage acquires a name and becomes historical to later show up as a savage visitor or member of different civ).
You might want to allow players to ban tribals and/or semi sapients from visiting the fortress to foster prejudice (already present versus goblins and elves among a lot of players).
Semi sapients are mechanically hard capped against commiting to labours they don't naturally already have skills, there's no real way around it & learn martial and non assignable skills at half pace. I have nothing against semi-sapients becoming mercenaries but i feel like it should be out of a sheltered society rather than spontaneously from the wild (EVIL entity ogre & troll merc's who specifically cater to babysnatcher / same kind of civs as them) due to their limited capacity to do pretty much anything beside punch stuff & haul (both good traits)
I don't see unicorns and dragons as viable in this context, as neither is considered sapient in DF. I certainly would want (non/low/normal/highly) sapient dragons though, and the same goes for other (semi) mega beasts. Once sapient these creatures would be suitable options for inclusion.
Since I've never gotten any animal people visitor who wanted to become a citizen or even resident, I have no experience of how well they function in the current DF version when joining the fortress though that avenue.
The dragon example is a extreme one, which i can agree the principle of semi-mega's having exclusive or restricted (/specific) access to certain roles should be considered. My point (though I foolishly forgot they dont both have intelligence) is about moddability and how additional modding might change things, it might be a bit impractical to have a fortress run entirely by non grasping unicorns (or hold everything inside their mouths) because your livestock taming got out of hand before everyone died in a tragic incident.
I can't remember either given that my DF sessions don't tend to last that long with a large amount of tavern traffic, If i remember correctly, they will adopt the dwarven CIV's ethics but retain whatever personal traits they have on their RAW that are hard wired into themselves. The should be accessible, but gremlins are not accessible because they are pets & the tameness is never cleared.
> I know from GM editor that trolls being part of the civ also posess power dreams relevant to the civ's values which they adopt, this is debatable whether this is intentional as it seems to quite buggy when put in perspective of semi-intelligents or all creatures that a indirectly part of your civ having sentient creature facets.