Umu is the closest thing to god that the beastmen had. He is, essentially, the first progenitor (much like Adam) in our world. He had a lot of children with his two nephew (it's a long story).
Unlike beastmen themselves, Umu is depicted as being quadruped. The beastmen worship for Umu is akin to ancestral worship.
Beastman and Human creation Story.
Trigger Warning: Mythological fuckups
In the beginning of time Earth Mother and Sky Father had a child named Air.
Sky Father attempted to have another child, but he didn't know that it took time for earth mother to heal. This attempted procreation made Earth Mother bled and created the sea we as know today. Air was angry that Sky Father wounded its mother.
Air ripped its father's virility and spat it onto the Earth. Sky's orbs turned into the Moon and the Sun while his seed impregnated Earth with animals. Air also drove father sky away so that he hovered far away from the earth.
Air made a human and a beast named Imi and Umu. After a while it saw that neither of them are bearing a child even after several attempts. Earth Mother told him that it would not work because both of them are men. Air tried to make a woman to rectify this and named her Ama. However... the woman it created had the body of a man and thus cannot bear a child. Air tried once again and he successfully created a neuter human that it named Eme.
For this reason humans have three sexes and need the cooperation of all three of them to create an offspring.
Making two more humans after Imi is beyond Air's limit. In breathing its essence into Eme, Air exhausted himself and lost the ability to think. Thus, Air couldn't make a female beast for Umu.
Imi, Eme and Ama had many children and their children in turn had many children. This made Umu feel lonely and jealous.
In order to have Imi back with him, Umu ripped Imi's virility and swallowed it so that Ama would not want him anymore. Imi however, did not despair and took an animal's virility and stitched it onto his body. Eme did not notice anything wrong until she bore another twins. What came out of her womb was half-man half-animal due to Imi's new virility. Ama, shocked at this revelation, hid the new twins from Imi. She delivered them to their uncle, Umu.
The twins are named Angku and Engki. They grew up to be a handsome beastmen and the consort of Umu. Angku and Engki satiated Umu so that he's no longer jealous of Imi.
The next time Eme her last children came up half women half beast. Ama named the next twins Hakau and Satau. Like their brothers, she delivered them to Umu.
Under the care of Umu, the sisters fell in love with Angku and Engki. However, Umu does not want to share Angku and Engki with the sisters. One day Hakau and Satau conspired one day to cut away Umu's virility so he could no longer lay with Angku and Engki.
They succeeded in drugging Umu and cut his virility. However they had a fight on who gets to use Umu's virility on Angku and Engki and they inadvertently killed each other in the feud. Griefed with the death of his niece Umu ate both Hakau and Satau, for it is the way the beasts honoured their family.
The next time he lied with Angku and Engki, Umu became pregnant. Umu was overjoyed because now he can have a family like the humans. Umu had many children from Angku and Engki and his children had many children.