A domestic animal that has never been in contact with humans will to a large degree return to a wild/feral state, albeit less aggressive overall than their wild ancestor. Chicks or poults etc. born locked away and left without any contact with dwarves should definitely become at least semi-wild or similar in my opinion, although it's hard to say if this is something intented or not in this instance without knowing the exact mechanics :>
The thing about domestication is that domestic species are typically entirely different species of creatures from their wild counterparts.
Species like corn, for example, look nothing like the original long grasses they were bred from. A cow would never survive in the wild, as it's been bred into being little more than a stomach on legs.
A chicken, likewise, would probably never be capable of becoming "wild", because its innate nature has been overwritten into being a creature that can only live on farms.
Dogs, you might make a case for them going back into being wild. Some of them, anyway, those tiny toy dogs are going to die, and probably so are the big water dogs and even the pit bulls, but the general "mutt" types of dogs are going to live and possibly return to being wild.
Still, it takes far more than a single generation to be able to do that, and anyway, all of that is beside the most important point, and that is:
[PET] creatures are meant to
always be tame. As in, Toady has said that they should never go untame, and that there will be a solid dividing line between the totally domesticated-over-centuries creatures that should never untame, and the exotics you tame that can never become fully and completely tamed, because that [PET] Token is an innate aspect of their own nature as a creature.