It appears that it wasn't William C. who married a Horry but that his grandmother was a Horry as shown below.Sending this because it can be documented from Joseph Davis' Will and other documentation.
Progenitor:Joseph Davis, ca:1760 Marion Dist. S.C. Married:1783; Anne, daughter of Buckingham and Elizabeth (Horry) Keene; Anne was born about 1758 and died about 1810/11.
Elizabeth (Horry) Keene's brother was Col. Peter Horry of S. C.(Peter Horry was a Rev. Soldier; right-hand man of Francis Marion) Elizabeth and Peter Horry's father was John Horry, who died April 10, 1770.Their uncle was Elias Horry.
Joseph Davis died in 1833 and his Will was probated November 14, 1833, Marion County. Children: Martha Davis ca: 1785 Abraham G Davis, ca: 1786 married Elizabeth ____ Eliza Davis, ca: 1787 Rachel Davis, ca: 1789; married:JamesNewson.She diedbefore 1850. Mar: June 7, 1813 Christina Davis, ca: 1790. Daniel H. Davis, ca: 1792 - 2nd wife?Charlotte T._____ WILLIAM C DAVIS, ca: 1794 Ann Maria Davis, ca: 1797 Benjamin S Davis, ca: August 17, 1798 mar: Susannah Mary L Davis, ca: 1801/2; married: James Thomas of Marlboro District; she was alive in 1850 Joseph J Davis, ca 1804 Susan B Davis, ca: 1808/9
***Also, provisions in his will were for Mary and Elizabeth Whaley, daughters of Aley Whaley.No explanation in Will.
Hope this isn't too much info.If I locate anything else I'll post it.