Gyumao is based on Niúmówáng, also known as "Ox-Demon-King," "Ox Devil Lord," or "Ox Tyrant," a character from the Chinese novel "Journey to the West."
Despite his intimidating name, he is actually quite harmless and fun-loving. Fans mistakenly believe that "Gyū" is his surname and "Maō" is his first name because "gyū" refers to the crest on his helmet. He appeared somewhat demonic and frightening when his castle was engulfed in flames on Frypan Mountain.
He caused chaos in the village at the foot of the mountain, threatening anyone he thought might steal his treasure until Kamesen'nin taught him the error of his ways. Gyumao is Chi-Chi's father and therefore Son Goku's father-in-law. He is very strong and trained with Grandpa Son Gohan under Muten-Rōshi's guidance.
As the series progresses, Gyūmaō's role diminishes and he eventually becomes a background character, used mainly for comic relief. Gyumao is also the grandfather of Gohan and Goten. In anime filler episodes, he occasionally has significant roles, primarily accompanying his family and others.
(Source: Wikipedia)
