Such names as Lyon, Bull, Buck, etc., may have been borrowed from armorial bearings, the shields and banners of war, or for a resemblance to those animals noted for courage, agility, or swiftness, or from signs and emblems over shops and inns.

Source: An Etymological Dictionary of Family and Christian Names With an Essay on their Derivation and Import; Arthur, William, M.A.; New York, NY: Sheldon, Blake, Bleeker & CO., 1857.