My transcript of the 1870 Coles County Illinois Census lists a Willis Matthews, age 6, along James, William, and Walter Dunn (ages 20, 18, and 16)in the Joseph Dunn household in Humboldt township.
I don't know if this just complicates the issue for you. I will try to find the Dunn household on the 1880 census when I get a chance.I didn't find a Willis Ferguson or Byrd Ferguson. It is possible that IRAD at EIU in Charleston has something about his birth or marriage.
If you found anything on Walter Dunn's horse racing legend, I would appreciate your sharing it.Tell me where you get with this puzzle.