Training takes little time as is. In addition, training is a binary matter. It never rusts and cannot be overridden or added to. Why not use the new teaching mechanic instead? Say, a Competent Animal Trainer could train a dog up to Competent in any relevant skill such as biting or sneaking. This would do away with the arbitrary distinctions between different types of trained animals. No more 2x damage boost for war dogs, instead the training would provide them with some muscle and better skills in various maneuvers. For herbivores, there'd be a skill for maintaining their cool. Such a beast would periodically roll a check against it's instincts when in battle to determine whether it panics and tries to flee or whatnot.
More intelligent animals would learn faster and their skills would rust slower. Every dog should have a cap on their stats based on genetics, avoiding even the litter runts being musclebound beasts with a little mannequin biting.
Mind you, I'd prefer if dog skills were simplified (along with simple dwarf skills but I digress):
- Dabbling
- Novice
- Competent
- Skilled
- Adept
- Master
Trainer's own skill would determine how well an animal can be trained. Every time an animal is assigned to be trained, the trainer's skill roll determines whether it can be trained further. Basically, let's take a dog that's an Adept Ambusher. An Accomplished trainer tries to train him further. He has say, 25% chance of success. If he wins the roll, the animal learns during the session, increasing it's skill towards Master. If he fails, the dog merely recovers from skill rusting. While this calls for some way of preventing trainers from wasting their time with animals they can't handle well yet, it'd make training take longer without having to be finished in one session and allows for further training if someone more skilled turns up later.
Trainer skill would have diminishing returns. While he would quickly learn to reliably train animals up to Competent, Skilled and beyond would be harder to reach. Animal's own traits would also come into play. Exotic (high difficulty rating) animals would require high trainer skill to make any progress with but once the threshold is reached, it'd go smoothly. Taming requiring some skill would also be preferable. Animals still in the child stage could have the modifier cut in half, making them much easier to handle. Or, just a different modifier for the different stages. Preferable, as it allows for adults to be unable to ever be truly tamed if so desired. Whatever works I guess.
A pretty crude draft but I think you get the idea.