Gyumao, also known as Ox-King, is the father of Chichi and thus, the grandpa of Son Gohan and Son Goten and, the father-in-law of Son Goku.
He appears first in the beginning of Dragon Ball, when Son Goku and Bulma are searching the Dragon Balls. They arrive to the castle of Gyumao that is involved in flames because the Fire Mountain has fired. The castle and the fortune of Gyumao is in Danger, so he decides to ask Son Goku to looking for Mutenroshi and beg for help.
Son Goku finds Mutenroshi, and the ancient master extinguish the fire, using a mistic skill known as Kame Hame Ha.
After that, Gyumao appears in Red Ribbon Sagas, and he is able to defeat a swarm of soldiers and tanks. He also appears in Piccolo Daimaku's sagas saying that Piccolo is invincible.
In Dragon Ball Z, he appears as a proudly father and grandpa, and visit his daughter and grandson as frequently as he can, but he is no more an evil villain, as he was known at the beginning of the story.