At the Battle of the Blackwater Rush,
King's Landing is being assaulted by Baratheon forces under Stannis. Tywin Lannister shows up leading his host from Harrenhal and most of Renly's army as well. They attack and rout Stannis's army from the rear. During the battle, many soldiers report seeing "Renly's Ghost" riding alongside Tywin Lannister and Loras Tyrell. The knight is described as wearing green armor very similar to King Renly's with golden antlers coming form the head.
Who is the knight wearing this armor? Is it ever revealed? Did such a knight even fight in the battle or was it a mass hallucination / popular rumor started by a few?