His ring name is the Spanish translation of Lion-O from the ThunderCats cartoon. He began his wrestling career working under the ring name "Mustang Negro" ("Black Mustang") but only used the name briefly from his debut in 2003 until early 2004. In 2004 he changed his ring name to Leono, the Spanish name for Lion-O of the ThunderCats, which was also reflected in the fact that Leono often had the ThunderCats logo on his trunks. Consejo Mundial de Lucha Libre (CMLL) created the "Guapos University" ("Hansome University"), or "Guapos U", a storyline mimicking realities shows such as the WWE Tough Enough show, searching for a new member to join Shocker, Máscara Mágica and El Terrible as part of "Los Guapos". The contestants included, among others,Leono, who became a regular worker for CMLL at this point. The group participated in a series of matches as they tried to determine a winner, including a tournament of Guapos U only contestants vying for a match for the Mexican National Trios Championship. The tournament was won by Misterioso, Jr., Brazo de Oro, Jr. and El Texano, Jr. although they were unable to win the title. During one of CMLL's shows El Terrible turned on the group and was joined by Último Guerrero in the attack on "Los Guapos".Sangre Azteca and El Koreano prevented the rest of the "Guapos U" class in making the save, leading to the two them being kicked out of the group. Guerrero immediately took both Sangre Azteca and Koreano under his wing, forming "Pandilla Guerrera" ("Gang of Warriors"), a mid-card group associated with Último Guerrero's "Los Guerreros del Infierno" group. The tournament ended with Alan Stone winning the tournament. He was one of 14 men competing in the 2005 Mexican G1 Junior Climax tournament, but lost to eventual tournament winner El Texano, Jr. in the preliminary "torneo cibernetico", multi-man elimination match. Leono was one of eight competitors to take part in the first ever "Reyes del Aire" ("Kings of the Air") tournament that took place on June 10, 2005. The tournament primarily featured younger, lower ranked wrestlers which besides Leono included Alan Stone, Ricky Marvin, Misterioso, Jr., Stuka, Jr., Tigre Blanco, Tigre Metálico, Virus and Volador Jr. Leono was the first man eliminated from the tournament.