Oh! Glad there's been some interest in this idea.
Some of the brought-up very specific real-world animals might be a little over-complex for Dwarf Fortress (hahahaha, but no, seriously now), and not worth it in Toady's eyes to model, so if there's going to be any chance of this idea being a genuinely viable suggestion to add onto Dwarf Fortress, then two criteria will need to be kept in mind, methinks;
1)It has to add some valuable functionality to the game
2)It has to be simple enough, programming-wise
The first objective was part of what got me to suggest this idea in the first place; over in the modding board, it was mentioned how things like cave floaters, floating guts, etc, make excellent punching bags but don't breed, even if you capture multiples of them. While it's not difficult at all to simply mod a female and male version of all such creatures, that's a somewhat crude solution to a more wide-affecting issue. It was even brought up that, because such things don't actually reproduce at all, someone said they don't do so during worldgen and it's theoretically possible that, like finite forgotten beasts and other megabeasts, they can eventually go extinct if the world history is run for long enough. But, I suppose on the other hand, if they were to actually reproduce during worldgen (or is that even simulated at all for wild populations? I dunno. SOmething to ask about further some other time, probably to start another topic about later as well) that may or may not cause issues with population dispersal and density and whatnot.
On a more significant topic and relevant to DF from all the mentions of various breeding schemes, being able to populate a DF world with creatures with more fantastical creatures, perhaps some with breeding schemes that don't exist in the real world, would help go a ways towards DF's stated goal of not being a generic fantasy game, but a generic fantasy world generator (if I am quoting that correctly). In one of the threads I tried to search for on this topic previously (the "new races" one), there was talk of something like a tribal race of lizard-people that worked with one alpha male and a harem of females, but also an entourage of beta males that don't reproduce.
But as for the second criteria, keeping things simple, this is where I can only hope someone with programming logic experience and familiarity with the tokening system in DF comes along to perhaps come up with a skeleton framework of tokens to suggest for use. But, with what little experience I have, what I mean by keeping things simple is we can't have tokens for every unique form of breeding there is, or even make any tokens specific to any -kind- of breeding. Rather, what we need is a bottom-up approach; a collection of small, simple rules that can be combined, mixed, and matched to form much more complex structures.
One approach I can think of for this is perhaps, in regards to reproduction, is to structure it in terms of "Input" and "Output"; what is
required to breed, and what
results from that breeding. Possible requirements could be [NEEDS_<something>_RESOURCES] (for things like, maybe algae-like life that only needs to eat something before growing and splitting off), [NEEDS_<other>_CREATURES] for various kinds of parasitic lifeforms that need hosts to breed (Xenomorphs and facehuggers, anyone?), and then [NEEDS_<something>_CASTE], for requiring another version of their own species (which is applicable to male-female standard reproduction schemes). There also might need to be some basic rules determining, perhaps, what kind of breeding takes place under what circumstances (some animals/creatures will produce more, or more more of a certain gender, if the population suffers some kind of it,), as well as for sentient races, societal considerations like "needs relationship of some kind", though with also sometimes some possibility of those societal considerations being less hard-set than the more biological ones (wedlock, anyone?). Something like hermaphrodites, I'm thinking would be too specific to program in on their own, and could just use a more general basic rule "just needs any one other of same species", with no separate castes.
Then there would need to be rules for what kind of offspring is produced, some of which I think is already present, like laying eggs, and how many are laid. But there also needs to be things like binary fission/mitosis, and things resulting from aformentioned parasitic breeding, as well as determining exactly what caste might result (sometimes influenced by the 'emergency breeding' conditions some animals have, like say, need more males/females/warriors/workers/other, both for dual-gender and multigender creatures) and what the chances are if there is chance involved.
On top of all this, there might also have to be an expansion to the 'transformation' system which, if I am not mistaken, has only just gotten some basics in with werecreatures. Some more control over what something can transform into, and when/under what conditions, would go a long way towards helping this system, like some transformations based on age (tadpole becomes a frog, maggot becomes an antman), and some transformations based on necessity (some animals will change their gender in response to a population hit, or change overtime).
Anyone good with programming logic feel like taking a crack at brainstorming up with a list of rules/tokens like that?
EDIT: Oh, and completely forgot the obvious; if Night Creatures could be brought under the umbrella of a general basic-rules reproduction system, that would be a major plus. I imagine that the primary way they "breed" wouldn't be related to reproduction at all; the part where they capture humanoids of the opposite gender of them and then transform that humanoid into a opposite-version of themselves would probably just be something like a very selective syndrome that has a transformation effect (maybe it doesn't even need to be modeled in-game at all), and they otherwise reproduce normally; "needs caste", which is applied to only females (whether that's the original Night Creature, or a male Night Creature's spouses), and where the needed caste is just males.