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!". He was voiced by Seth MacFarlane.

In "Peter's Two Dads", Thaddeus is not mentioned. In "Padre de Familia", Peter was born alone, with Thaddeus nowhere to be seen. In "Brothers & Sisters", Peter states that he doesn't have a brother or a sister, but instead a broster, half-brother, half-sister.