You searched for: “agnomen”
agnomen (s) (ag NOH muhn) (noun); agnomina (pl) (ag NUHM uh nuh), agnomens
1. A fourth name, or a nickname, presented by Romans; an additional name subsequently acquired as an honor for some special achievement; such as, military victories: "An example of a Roman agnomen is Publius Caius Scipio Africanus (Scipio the Elder or Scipio Sr.) who was a hero of the Punic Wars and victorious over Hannibal."
"Ferdinand the Great is another example of an agnomen which has been used since the Roman times."2. Etymology: from Late Latin ag-, a form of ad-, "toward" ["addition to"] +nomen, "name".