Ida Davis (formerly Daniel "Dan" Quagmire) is Glenn Quagmire's father from Family Guy. He is transsexual because he undergoes surgery to become Ida, much to his son's chagrin. He/she is voiced by Seth MacFarlane.
Daniel Quagmire is a U.S. Army was veteran that fought in Vietnam, serving as a war pilot. When he wasn't fighting, he and male armymen took part in sexual activities with each other. During this time, he came to grips with the fact that he was actually a woman in a man's body.
It wasn't until the events of the episode "Quagmire's Mom" that he told his son, Glenn Quagmire about this. It was not easy for Glenn to adjust to this, because it was in the Quagmire family name for the men to act like ladies men, so this was kind of embarassing to him. However, Quagmire supported this and was happy to have Dan follow through with his transition into Ida Davis.