As far as I know 'wild' creatures will never become tame but you can advance the training level over generations, so a semi-wild creature may have 'trained' children and so on.
I'd love to know if this is the case - anyone else see this? No reason why it's not, and curious to see. Gotta get my cave crocodiles breeding then, while they are in their semi-wild state.... hmmm. To the croc-lab.
Giant cockatiel chicks hatched with the mother's training rank (exceptional, triple bar), fell back one step in training to the probable father's level, but now seem to be stable at that * and aren't losing anything else. So far, so good. The only hard part was limiting access to the eggs.
Giant rats bred exceedingly well and quickly, but there was a rat tantrum spiral when one of the mothers reverted and all the semi-wild young escaped in the chaos. All rats had to be euthanized to stop the madness, either with the military or at the butcher's shop.
The goblins also brought me a custom snail as a mount, which was considered domestic and easily fully trained. It sat peacefully guarding my entryway for a while with no signs of any problems. Unfortunately, it attacked a human caravan on sight and had to be euthanized. I suspect training only erases the creature's grudges against the civilization that trained it and it was still an enemy of the humans. I think this bug could be exploited to have a crocodile pond or evil snail pit that's harmless to dwarves but lethal to unwanted traders.