Ox-King, also known as Gyumao, is a supporting character in the Dragon Ball series and the ruler of Fire Mountain, also called Mount Frypan. In his youth, he was a bandit king who conquered the mountain fortress that became his kingdom, and he trained as a martial artist under Master Roshi at the Turtle School alongside Grandpa Gohan, the adoptive grandfather of Goku. That training gave him a solid foundation in fighting technique and considerable combat skill for an ordinary human.

He is the father of Chi-Chi and later becomes the father-in-law of Goku and the grandfather of Gohan and Goten. Early in the story, his castle becomes engulfed in magical flames during a picnic with his daughter, trapping him and his fortune atop the mountain for years. During that time, he grew frightening to the people in the village below, attacking anyone he suspected of trying to steal his treasure, and earned the nickname Emperor of Demons. When Goku and Bulma encounter him while searching for the Dragon Balls, he pleads for help, and Goku brings Master Roshi, who extinguishes the flames with a Kamehameha wave that destroys the mountain and castle along with them. As a gesture of gratitude for the safe return of his daughter, Ox-King offers Chi-Chi to Goku as a wife, a promise that eventually leads to their marriage.

Despite his enormous size, horned helmet, wild beard, and fearsome reputation, Ox-King is gentle-hearted, good-natured, and deeply devoted to his family. He is a doting father who becomes tearful at the thought of his daughter's safety, and his main motivation throughout the series is the wellbeing and happiness of his loved ones. His role in the story is largely domestic: he provides financial support to Goku and Chi-Chi's household when the family struggles, cares for and dotes on his grandchildren Gohan and Goten, and appears at family gatherings and tournaments to cheer for his relatives. Early on he also shows his strength by defeating squads of Red Ribbon Army soldiers and tanks, and during the King Piccolo conflict he is marked for elimination as a known martial artist before being saved by Goku.

Over time, Ox-King develops from a menacing bandit king into a peaceful family man. He gives up his warrior lifestyle, exchanges his armor and goggles for ordinary clothing and glasses, and becomes a recurring presence in the later sagas. In the Majin Buu arc he takes refuge with other survivors, is turned into chocolate and consumed by Buu, and is later revived along with the rest of the Earth's population. In his final appearances, he lives into old age and remains present for the youngest generation of his family, including his great-granddaughter Pan.

In terms of abilities, Ox-King is exceptionally strong for an ordinary Earthling. He stands over four meters tall, can lift massive boulders, and wields a giant battle-axe with one hand. His personal fighting style, known as Ox-King Style, combines heavy axe strikes with the fundamentals of Turtle School training, and he passed this style on to his daughter Chi-Chi. His years of surviving atop the burning mountain also gave him remarkable endurance and resistance to heat. He cannot fly or use energy attacks, and while his strength is formidable against ordinary opponents, it is far below the level of the series' greatest warriors and gradually declines as he ages and settles into domestic life.