Thaddeus "Blacktooth" Tannen was the uncle of Buford Tannen (who would terrorize Hill Valley in 1885, ten years after Thaddeus bullied the Parkers). Like many of the Tannens, Thaddeus was a bully and the leader of his own gang. Thaddeus had a sister, Hepzibah Tannen, who later married one of the members of that gang. In 1875, he had swindled the Parker family out of their property, and 116 years later, Biff Tannen claimed to have inherited the land from Thaddeus.
After Biff found the deed from Wendell Parker to Thaddeus Tannen, and made claim to the Parker Ranch, Marty, Jules and Verne traveled back to the 1875 to right the wrong. It turned out that Mr. Parker's signature had been made under durress, with Thaddeus threatening Genevieve Parker. Marty allowed Thaddeus to believe that a deed to the ranch had been obtained. However, when Biff unearthed the deed in 1991, it no longer had a signature, and the Parker ranch remained with the Parkers.
It is unclear what happened to Thaddeus Tannen after 1875, but he never got around to using the deed after hiding it in the ground.