I it possible for a particular strain to become extinct? Say if the last member of a particular strain died before spreading the infection, and the relevant god wasn't widespread enough to curse someone else?
Is it possible to get like a snowball effect, if one strain of werecreature is wildly successful or if it's in perfect enviromental conditions?
I use Legends Viewer to examine the werebeast historical figures when I'm working on generating a new world. From what I've seen there usually is one type of werebeast that's much more prolific than other types. I'm not sure what causes it but what I typically see are between 2 and 5 at most werebeast types which are actually represented by having an individual cursed at some point. Of these one or sometimes two is over represented in the population. There doesn't seem to be any pattern to what types of creatures dominate either, a werelizard is as likely to win out as a weremammoth at least as far as I can tell.
My best guess is that profaning a temple doesn't actually happen that often so only a few werebeasts are made. Of those some will die off and not spread others will rapidly spread, it seems random based on however the game handles those fights in world generation. My results are typically something like four werebeast types actually become reality in the world, one dominates potentially producing hundreds of living specimens, one goes extinct and the other two will have a small handful, sometimes even only a single living specimen.
However I don't think a werebeast type can become permanently extinct, you just have to keep rolling until the curse is of the desired type either by savescumming or having lots of test subjects. You can use legends or legends viewer to identify a god which has cursed someone for profaning it's temple in the past. That god will always curse someone for profaning a temple in fort mode and it has access to the full list of potential curses for your world. One temple, a dozen statues and a dozen subjects could result in a dozen werebeasts/vampires.
It might be possible, I haven't tested it yet but I think I might, to produce werebeasts in your fort and send them out into the world. A wild or invader werebeast troll released off the map for example should be a historical figure and therefore potentially spread the curse. You could also infect your dwarves and banish them from your fort sending them out into the world to do the same. You could potentially resurrect or introduce new curse lines this way.