Stan Thompson is possibly Meg's real father according to a scene in "Screwed the Pooch".
Attempting to persuade the court that Brian should have access to his puppies, Peter observes that if he were half the dad that Brian is he would know certain facts about his children, including the fact that "Meg's real father is..." Brian answers, "Stan Thompson." The irony is that Meg does not hear this revelation as she is listening to music on headphones at the time.
Brian later mentions in "Not All Dogs Go To Heaven" to Meg of how she resembles Peter. This either could be to refute the statement of Stan Thompson being Meg's real father, or that Brian may have been lying to talk Meg out of burning "non-christian" books.