I think i remember 1 time my dad rang up the RSPCA to say there was some horse near our house that appeared mistreated.As far as i remember they cared **** all about it.
Aside from the chickens there's already one permanent pet in the house: the goldfish. Discussions suggest that none of the housemates really care about it.
I can suggest some reasons why the house might not be considered a healthy environment for Paddy. He was there as a task object and couldn't be guaranteed the love and respect that he'd get from his human. Being separated from his "pack leader" for long periods could also cause some anxiety, which could be exacerbated by an alien environment that contains few familiar objects and none of his human's smells. Remember that he's not a puppy but a full-grown adult with set habits and routines. On top of that, there's also the feelings (and schedules) of his human family to consider.
in the australian version of BB, the housemates were given a dog. she only lasted a couple of days before the HM's decided it was better for her to leave.
recently though, they were given a Sony Aibo dog to play with, and to train as part of a task.