Meg Griffin (biological daughter)
Stanley "Stan" Thompson is the biological father of Meg Griffin. He and Lois Petwerschmidt gave birth to her on accident, he did not want to support her, so he left her with Lois, who raised her with the adoptive father, Peter Griffin. To this day, Meg is unaware of the fact that she never knew her real father, although everyone else in the including family, including even Chris and Stewie, does.
Lois got drunk at a party and had sex with Stan Thompson, whose condom broke, impregnating her. At the time, Lois was married to Peter and they were not planning on having a child. Stan abandoned Lois, not wanting to take the responsibility. Eventually, Lois had the baby, and that's how Meg was born. Lois and Peter raised Meg and decided it'd be best not to tell her about Stan. They did, however, tell all of their other kids about it, including Brian, but after all these years, Meg is still oblivious to this fact.
His first mention was in "Screwed the Pooch", where Brian was showing how much about The Griffin Family he knew, by finishing all of Peter's sentences. He mentioned Chris' favorite ice cream flavor, Stewie's favorite bedtime story, and Meg's real father. When mentioning Meg's real father, though, Meg was listening to music on her Ipod, so she did not hear any of it.
In "Not All Dogs Go To Heaven", Brian was arguing with the rest of the family about religion. He said that one thing that would be different if there was a God was Meg would know about Stan Thompson. The rest of the family shushed him, not wanting her to know about it.
In "Peter's Two Dads", Peter learned that his biological father Mickey McFinnagin knocked up his mom and never called her again, leaving her with custody of himself and having him grow up with an adoptive father, who he thought was his real dad. He compared this concept to the completely identical situation that involved himself, Meg, Lois, and Stan Thompson.
In "Meg Stinks!", Peter was actually being nice to Meg and treating her like a friend. He told Lois a list of things he was going to do with her before she stopped him from the charade. One of those things was telling her about Stan Thompson.