Thaddius Riker was a human who lived on 19th century Earth. Thaddius served as a Colonel in the Union Army during the American Civil War. Thaddius was a distant ancestor of Kyle Riker, and Kyle's son Captain William T. Riker. Thaddius had the nickname of "Old Iron Boots." He commanded the 102nd New York Infantry during General William T. Sherman's March to the Sea campaign.
During the Battle of Pine Mountain, Georgia in late June 1864, Thaddius Riker was seriously injured. Thaddius was carried back from the front line by another soldier. Over 500 years later, in 2372, it was discovered that the soldier who carried Thaddius back from the front lines was Quinn, a member of the Q Continuum. It was further discovered that without this Q's assistance, Thaddius most likely would have died in the battle, and not have had any descendants. (VOY episode: "Death Wish")
When Riker believed himself to be dying, he asked Quinn to deliver a message to his family. Quinn fulfilled the promise in 2371, to Riker's descendant William T. Riker. (TNG short story: "A Q to Swear By")