Thaddeus Griffin is Peter Griffin's evil twin brother. He shows up in "Mother Tucker" when Peter's mother announced she has left her husband Francis Griffin. He declares this new event will surely affect his inheritance. He has a habit of rubbing his mustache between his fingers and saying "Nyah!".
Thaddeus is voiced by Seth MacFarlane.
In "Brothers & Sisters", Peter states that he doesn't have a brother or a sister, but instead a broster, half-brother, half-sister.