Well, I started on this quixotic quest here:
Breeding Beak Dogs, but it has moved into modding territory.
Summary:
1) I added "TRAINABLE" and "PET" to Beak Dogs.
2) I captured 20 or so in cage traps. They were "Invaders" on the "u" menu.
3) I threw them into a pit containing all of my cats, four dogs, two donkeys and a horse.
4) The horse kicked the everloving stuffing out of them. But I managed to isolate the horse. So...
5) I did indeedy get "Beak Dog pups". Excellent!
6) Um, the other beak dogs don't like them. The pups are initially "hostile" on the "u" menu.
7) I cage everyone, sort them, kill the older dogs off.
8) I'm able to tame the pups! I get "Stray Beak Dog Pups (Tame)" pretty much on-plan here.
9) The pups grew up.
10) The Dungeon Master says no suitable animal when I ask for a War Critter.
11) I let -this- generation breed.
12) I kill off the first born-in-captivity generation.
13) I let the remaining never-hostile pups grow up.
14) They are indeed trainable into War Beak Dogs.
Much celebration.
But...
An unlucky kobold snuck in my sally port and met a war beak dog. Suddenly my sally port has a liberal coating of blood... and my dwarves are cowering from my war beak dog. Very irritating. He's still marked as tame.
The butcher was able to walk right up to him and take him for a little one-way walk without ever getting scared (which is weird too).
But... why did this happen? And is there a tag I can change to prevent this without regenerating the world? All I've modified is adding "PET" and "TRAINABLE" to stock beak dogs in an existing world. The goblins provided the beak dogs (in the hundreds by this point. I have 300 beak dog quivers, for instance.)