In 1955, Harley Clark was the head cheerleader at Texas. At a pep rally the night before the Longhorns hosted TCU, Clark decided to introduce a hand sign that he and his friend Henry Pitts had devised. Clark and Pitts had concluded that a hand sign with the index and little fingers extended resembled a longhorn. They thought this would be a good idea to introduce as a universal rallying symbol, similar to the "Gig 'em" thumbs-up sign at Texas A&M.
Clark took the stage at the pep rally and decreed: "This is the official hand sign of the University of Texas, to be used whenever and wherever Longhorns gather." Since then, the "Hook'em Horns" sign has become synonymous with the school. Here's the man, who got it all started: