Thaddeus (Thaddious) "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. He earned the nickname "Blacktooth" after Marty pranked him with trick gum. While Jules and Verne Brown were in town, they saw wanted posters of Thaddeus, his other member Ox "Black Lips" McPhips and Marty "Black Eye" McFly. Marty most likely got his black eye by Thaddeus hitting him after being pranked.
Thaddeus had a sister, Hepzibah, 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. Thaddeus also appeared to have trouble spelling. When using Marty's back to sign his name with an invisible ink pen he claimed to "Forgot to dot the I". When Marty said there wasn't an I in Thaddeus, Thaddeus noted "There is now."
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 duress, 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.
But he never got around to using the deed after hiding it in the ground. Since his sister Hepzibah turned him into the authorities and he is in jail.