Yeah, I'm pretty jazzed about the way this sounds like it's going to shape up. I'm guessing civilizational knowledge will be generated in part by the simulated interactions between the races and the local wildlife, so a dwarven civ in the desert might use tamed giant scorpions as mounts/draft animals, but be utterly stymied trying to tame a muskox. Or the aforementioned polar bear cavalry (although dwarves need Riding before that can occur). Or underground civs using a LOT more exotic mounts.
Also like the suggestion someone made of having some of that knowledge be available in books (
How to Tame Your Rutherer: A Guide) which could be traded, captured, lost, etc. Said books could also function as a way of preserving some level of skill so that if your High Master Animal Tamer meets a gruesome end, future tamers could read some of his books and learn some of the lore rather than having to train up from scratch. (Actually, books could work that way for a LOT of skills...)
Jungle elves will suddenly be fun trade partners ("Okay, here's your annual shipment of tamed Bengal tigers").
Back to the cavalry, it would be really nice if Riding and mounted combat get added in as options for fortresses and other civs widely. Elven mounted archers riding unicorns, human civs that use camel lancers, the Dwarven Honey Badger Uhlans...
Perhaps the [MOUNT] flag could be made size-dependent as well. So dwarves couldn't ride horses, but they could ride smaller creatures (wild boars are always a popular choice). The nice thing with doing it that way is that everything scales for modding. If you have pixie civs, they could ride mosquitoes or something. If you have giant civs, they can ride rocs or dragons or something.