Gyumao is based on the Ox-Demon-King from Journey to the West, though despite the intimidating name, he's actually pretty laid-back and good-natured. The "Gyū" part comes from the ox symbol on his helmet—fans often mistakenly think it's his last name, with Maō being his first.
He wasn't always so chill, though. When his castle caught fire on Frypan Mountain, he went full demon mode, terrorizing the village below and obsessing over his treasure until Kame-Sennin set him straight. That's when things changed for him.
He's Chichi's father, making him Goku's father-in-law, and he actually trained under the same master as Goku's adopted grandfather—both studied under Muten-Roshi. Pretty strong guy in his own right.
As the series goes on, he basically becomes background noise, showing up occasionally for a quick joke. Same deal with the anime filler—he tags along with the family and crew from time to time. He's also grandfather to Gohan and Goten, though by that point he's pretty far removed from the main action.
Content compiled by AnimeList.moe from publicly available sources.
