Horn announced that he was beginning preparation for the NFL draft on Monday. Mukuamu, whom ESPN's Todd McShay ranks as the No. 4 cornerback available, announced his decision on Tuesday.
ESPN's Mel Kiper Jr. lists Horn, the son of former NFL wide receiver Joe Horn, as the sixth-best draft-eligible cornerback. Horn leaves the Gamecocks with the most interceptions (2) and pass breakups (6) on the team this season.
South Carolina fell to 2-5 after a loss to Ole Miss on Saturday.
The Gamecocks, who host Missouri this weekend, have three remaining regular-season games.