Interex
The Interex was basically what the Tau think they are.
The Interex, probably the closest thing we can glimpse about Human Civilization during the period of enlightenment known as the Dark Age of Technology, was made up of technologically advanced humans, but also alien races like the kinebrach, which were like space gorillas. Interex ships were bigger and more advanced than those of the Imperium, and they actually used AI and robots. While they were capable of wiping out hostile xenos, they felt it was better to just scale them down to one planet surrounded by warning satellites and make the hostile race incapable of spaceflight. They were aware of and fought against Chaos, which they called Kaos, unlike the Imperium at the time, which officially denied the existence of daemons. The Interex language is augmented/accompanied by some kind of music based on universal math called the aria, but they are not prissy. Their warriors could combine with four-legged robots to form a centaur-like warrior that fought with laser arrows. These warriors were almost as good as an Astartes. They felt that war was bad though, and this idealism would lead to their replacement by the Imperium as prime galactic power.
In the first Horus Heresy book, Horus meets them and decides to actually negotiate with them, because he feels that the galaxy is too grimdark and the Imperium purges things too often. When it seems that everything is going well, and the Imperium might just not suck under Horus, Erebus steals a kinebrach sword called an Anathame (which was related to Nurgle), blows up a war museum called the Hall of Devices, and starts a war. Horus is scratched, and sulks off to feel bad, while the Interex are mentioned to have been purged in an offhand mention in the next book.