The Beastman Country was founded by an unknown beastman at an unspecified time. Some before the Massacre at Katze Plains, the Commonwealth invaded the Great Minotaur Nation.
Overlord Second Half Arc
The Beastman Commonwealth was mentioned by Rangobart Eck Waria Roberbad during his inner dialogue as he admired the Death Knight under the command of the Archduke Ainz Ooal Gown. He compared them to the golems used by the Commonwealth.
So far its cultural aspect has yet to be touched upon.
Since the Commonwealth is located in the central continent, a region where demi-humans regard humans as slaves or food, it possible that this nation holds these views. On the other hand, there are reports that the Commonwealth has taken aggressive stances on its neighbors.
In the Web Novel, the Beastman Country once used their golems to launch an attack on the Great Minotaur Nation. The battle with the minotaurs caused the Beastman Country to lose four of its eight centaur bodied golem cavalry.
It's mentioned that the Commonwealth is famous for possessing 8-meter centaur-type golems. Golems are seen as decisive weapons for their country. If it was not for the weapons left by the so-called Minotaur Sage in the Great Minotaur Nation, the Beastman Commonwealth may have been able to destroy that nation with a few of their golems. They were Golems of that strength which possesses overwhelming power.
- According to one of Maruyama's tweets, more than just one "Beastman Country" alone exists in the New World. Because of that, there are several countries of beastman just like there are several human countries. The Beastman Country originated outside of the mainland continent. Moreover, the country of beastman attacking the Dragon Kingdom is different from the country of beastman in the center of the continent.