Who was inside the green armor with stag horns in the Battle of Blackwater?

Question

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?

Answer 1

It was Garlan Tyrell. Loras Tyrell confesses it to Jaime Lannister during their conversation in the White Room of the Kingsguard.

From A Storm of Swords, Jaime's POV, page 759:

"It's said you fought magnificiently in the battle... almost as well as Lord Renly's ghost beside you. A Sworn Brother has no secrets from his Lord Commander. Tell me, ser. Who was wearing Renly's armor?"
For a moment Loras Tyrell looked as though he might refuse, but in the end he remembered his vows. "My brother," he said sullenly. "Renly was taller than me, and broader in the chest. His armor was too loose on me, but it suited Garlan well."
"Was the masquerade your notion, or his?"
"Lord Littlefinger suggested it. He said it would frighten Stannis's ignorant men-at-arms."

It is also mentioned on the wiki, although it is not easy to find: http://awoiaf.westeros.org/index.php/Renly_Baratheon#A_Clash_of_Kings

Answer 2

The answer is contained within A Storm of Swords:

"Tell me, ser. Who was wearing Renly's armor?"

For a moment Loras Tyrell looked as though he might refuse, but (...) "My brother," he said sullenly. "Renly was taller than me, and broader in the chest. His armor was too loose on me, but it suited Garlan well."

"Was the masquerade your notion, or his?"

"Lord Littlefinger suggested it. He said it would frighten Stannis's ignorant men-at-arms."