Some food for thought derived from the book Guns, Germs, and Steel (which everyone should read!!!)...
Perhaps there ought to be a distinction between "tame" and "domesticated".
Any intelligent animal ought to be able to be tamed and possibly be able to be trained. The conditions that would dictate the ease of this would be something along the lines of the animal's age (and state of psychological development), skills, attributes, and temperament of the animal, as well as skills, attributes, and temperament of the person trying to tame said animal. In many cases, it would be practically (not necessarily absolutely) impossible. Think about real life: people have tamed and trained all manner of critters, ranging from crocodiles, to zebras, to elephants, to hippos!
Domestication is another matter... Not every animal should be able to be domesticated. What I mean by domesticated is bred in such a way that the animal will pretty much be guaranteed to have the disposition from birth and throughout its life to not only tolerate humans (or dwarves in this case), but consider them practically one of their own (or at least not generally view them as a threat, both physically and socially) and allow for people to mettle in their affairs and not be so bothered by their influence that they cannot breed and exist contently in captivity. Some animals are just too plain mean, or solitary, or skittish or what-have-you to be realistically domesticable. Let's compare zebras and horses (an example from Guns, Germs, and Steel). The ancestor of the domestic horse was calm enough to be approached by humans at some point to allow them to share space with them and even interact with them. This allowed for humans to develop a relationship with horses that was beneficial for the survival of those ancestral horses that were willing to hang around people. Over many generations this selected for horses with the correct disposition, and over time, horses became domesticated. Zebras, on the other hand, have never been able to be domesticated despite humanity's best efforts (and believe me, we've tried!!!). They and their ancestors were too mean spirited and distrusting, which precluded them from ever going through the same scenario as horses did. They would never tolerate being herded and bossed around by people. I hope this makes sense. Hippos would fall under the same thing as zebras. They can be tamed, but are not domesticable in the sense I mean.
So, in game terms, what would this mean?
Say you want to have an army of war hippos... If their temperaments and natural behaviors were modeled, it would pretty much play out like this. You'd have to capture wild hippos and train those that are right at the right age (like people have done with elephants since time immemorial). You could try to capture an adult hippo, but due to parameters dictated by age, attributes, etc., the dice will pretty much never roll in favor of it being tamed and trained. You could also try to breed those successfully tamed ones in captivity and try to have a herd of them fenced in, but low birth rates combined with the risk having a male who is aggressive during mating season, or a female protective of their young, etc. going on a rampage killing your dudes would make such an industry be way more trouble than it's worth (though !!FUN!!), at least in my opinion, which is probably why you never saw war hippos in real life (at least I think). So, in game, you could probably get several war hippos if you are lucky, but making a breeding program to make a huge army of them would be VERY hard.
The way I see it, you should be able to train any intelligent animal you want, without some sort of hard coded restriction, which I think would be just fine. Leave it up to the player to decide if their endeavor to train a ferocious herd of war-llamas is worth the effort and find out what happens when sent against the goblin horde...
The way I would see such a thing being able to occur in game as a sort of emergent phenomenon that isn't scripted hard-core is have it so that the raws for an animal could have tags that give them certain behaviors and traits (like being aggressive during mating season, being protective of young, being generally skittish, social structure (think wolves vs. tigers vs. cows), hunting behavior, pedomorphic behavior throughout life, etc.) that would dictate (emergently) how the player could interact with them (like the hippo example I mentioned earlier). So, it wouldn't be a matter of one animal having a simple [DOMESTICABLE] tag and the other not... it would be dictated by their behaviors and player-animal interactions.
That would be neat emergent behavior goodness, without the need to explicitly define whether or not an animal is trainable, tameable, etc.!!!!
For a better explanation of the premises for my post, read Guns, Germs, and Steel which has an awesome chapter about what makes a given plant or animal domesticable!
It's really frikkin' neat and definitely applicable to this game!!! Oh, goodnes... this gets me to another idea pertaining to world gen and a way to procedurally determine what animals and plants a civ might use... but I think that's sort of beyond the scope of world gen...