That would be James Ervin Spivey.
His comrades nicknamed him: "the 26th Georgia's Bull " .
The Yankees called him "Gordon's Bull"
James Ervin Spivey, Company E (Wiregrass Minutemen), 26th Georgia was famous in both armies for his loud battle cry. "Just as we were ordered forward, Irvin Spivy [sic], of the Twenty-Sixth Georgia Regiment, hallooed. He could halloo the queerest that I ever heard any one. It was a kind of a scream or low, like a terrible bull, with a kind of neigh mixed along with it, and it was nearly as loud as a steam whistle. We called him 'The Twenty-Sixth Georgia's bull," and the Yankee's called him "Gordon's Bull." He would always halloo this way when we charged the enemy, and we were informed that the Yankees understood it as a signal for them to move back." Recollections of Pvt. G. W. Nichols, 61st Georgia. Spivey enlisted as a private on April 22, 1861 and was appointed 4th Corporal on May 10, 1862. He was wounded in 1864 but returned to ranks to surrender with his unit at Appomattox Court House.
Main: a soldier in the 26th Georgia, James Ervin Spivey, who was famed in both the Army of Northern Virginia and the Army of the Potomac for his battle cry. "It was a kind of scream or low, like a terrible bull, with a kind of neigh mixed along with it, and it was nearly as loud as a steam whistle." He was known as the 'Georgia Bull', by his comrades, and 'Gordon's Bull', by the Federals.
GENERAL CUSTER'S FAREWELL ORDER.
Headquarters 3d Cavalry Division,
Appomattox Court House, Va., April 9, 1865.
Soldiers of the 3d Cavalry Division,
Source - Camp and field. Sketches of army life written by those who followed the flag. '61-'65
Edit - In addition to "Twenty-sixth Georgia's Bull," I am also accepting "Georgia's Bull" (without the "26th") and "Gordon's Bull" as correct answers to the part of the question that asked what Spivey's comrades called him. Both of these have been supported by sources provided by the players.