Embers in the Night Sky
There are countless alien species in the galaxy between the Humans and the Vaylen. The two most common are the Kerrns and the Mukhadish, both former Vaylen slave races, but there are others too.
The Kerrns are eight-foot-tall bipedal lizards. Genetically engineered by the Vaylen to serve as slaves to perform manual labor, the Kerrns provided too well made. The Kerrns immune system developed the ability to reject Vaylen hosts and, thus, the Kerrns where able to rise up and rebel against the Vaylen. While unable to defeat the Vaylen, the Kerrns where able to steal Vaylen craft and escape Vaylen space.
After fleeing the Vaylen many Kerrns settled among the Humans of the Iron Empire where they were able to find work as laborers, soldiers and bodyguards. More the Humans, the Kerrns are totally committed to fighting off the Vaylen and any Kerrn on a planet present an obstacle for Vaylen conquest. Despite this, most Kerrn prefer to live simple lives as laborers.
While best know to Humans for their strength, the Kerrns are highly intelligent. Without a world of their own, the Kerrns combined their stolen Vaylen spaceships into an artificially homeworld for themselves. The majority of the Kerrns still live on their hidden homeworld away from the Vaylen threat.
After the Kerrns rebelled, the Vaylen needed a new slave species so they created the Mukhadish. Larger but dumber that the Kerrns, the Mukhadish are a much better slave who are still controlled by the Vaylen to this day.
This is not to say that all Mukhadish are controlled by the Vaylen. In worlds that have successfully fought off a Vaylen invasion, Mukhadish are often left behind during the retreat. The result is that on such worlds, wild Mukhadish can be found roaming free in the countryside.
Despite their size they not particularly aggressive or dangerous but they are easily manipulated. Because they are such loyal servants, Mukhadish bodyguards and slaves have become popular in the criminal underworld.
The Kerrns hate the Mukhadish and (along with everything else they associate with the Vaylen) the Kerrns will attack them on sight.
There are countless other aliens but they fall into two general categories. The first category is that the aliens may be the product of other genetic experiments conducted by the Vaylen. The second category is that many worlds have given life to other exotic beings, some of which are also sentient. In either case, such species tend to be much rarer then the four main species.