I found this too:
"Despite tales involving a 1914 game against Carlisle, a 1913 pep rally at Langdon Hall, or an Osage Indian chief temporarily settling his tribe near campus, the favorite story of the AU faithful, regarding this battle cry, centers around the first ever football game against rival Georgia in 1892. The story goes that a spectator, who was a Civil War veteran, brought with him his pet eagle, which he had found on a Civil War battlefield 30 years prior. Witness accounts indicate that the eagle broke free from his master and began circling the stadium at the same time the Tiger offensive unit was marching down the field. The Auburn fans and students began yelling War Eagle! to cheer their team onto victory. The Tigers prevailed and, as the contest ended, the eagle flew downward, crashed into the ground and died. From that day forward, the War Eagle! battle cry was used to honor the fallen bird."