Leo m. Latin leo, Ancient Greek λέων 'lion'.
The name of 10 pre-1600 popes, six Byzantine emperors between the 5th and 10th C, an 8th C saint, and five kings of Armenia between the 12th and 14th C. The diminutive Lionel was the name of a younger son of King Bors and Evaine in the medieval Arthurian legends.
The name was often used by Jews as a translation of Hebrew Arye.