Why Stan can talkHe's not really a dog- he's a Melmacian, rocketed away from his dying planet as an infant, with no knowledge of his extraterrestrial origins. That would explain his interest in eating cats and ability to write/type, as neither of those things are really doglike traits. As to why he normally walks on four legs, he may be a subspecies that never achieved bipedal status.
Ellen and Bennett will find out Stan can talk in the last episode.
Ellen and Bennett will find out they've met before.