The Mutaren are an extraterrestrial species visibly distinct from galactic humans from the get-go. Their skin is a range of blue to green, leathery and heavily contoured.
Their features greatly resemble the scaled lizards of Sirian Planet - nearly all Mutaren have elongated, fanged jaws and clawed hands and feet. Less ubiquitous are secondary features like crests, tails, pronounced dewlaps, horny protrusions along the brow ridge and coloration changes like spots or stripes.
An early space-faring race in the Rutaren Galaxy the Mutaren expanded quickly, becoming known as warlike throughout the local sector. This impression is due largely to the fact that the first Mutaren in space were also the most militaristic nation from their homeworld. The drive for war sparked an arms race that resulted in viable space flight and habitation - so it was the most warlike Mutaren who left home and were encountered by other species.
Despite their physiology, they are a capable tool-using species with technology that rivals most other species in the galaxy, largely due to regular scavenging efforts across the worlds they've conquered. Reclaimed Zuxian tech and other local innovations combined with a variety of unusual combat tactics allowed them to largely fight the technologically-superior Juren Empire to a standstill.
The common name 'mutaren' was given to them by the Gargorians as a pun on 'Rutaren mutations' after early encounters mistook them for warped humans, rather than a genetically distinct species. The term was carried with the Gargorian Colonists to Sirian planet, where it caught on locally. What the Mutaren call themselves does not have an easy parallel in human speech. Juren largely refer to them simply as 'the enemy' or a variety of bestial slurs.
The Mutaren Confederation is something of an ironic name, as it refers to the Mutaren galactic military and the hundreds of populated worlds they've conquered and claimed as their own. The conquered peoples that the Mutaren do not exterminate are often taxed under threat of death for a substantial portion of local planetary minerals or technological innovations that can help the Mutaren military remain superior.
Despite impressive holdings in the galaxy, the Mutaren seem hellbent on expanding their influence and conquering all other species they encounter. The impetus for their expansion comes from a partial reliance on scavenged tech and the ongoing conflict with the Juren Empire leading to a regular need for an 'edge' in military encounters.
Additionally, first contact with the Juren Empire a millennium ago occurred shortly after a local schism within the Juren Empire (then the Zuxian). Encountering the militaristic forerunners of the Mutaren, the embattled, xenophobic Juren laid waste to early emissaries and launched a brief campaign pushing into Mutaren space. This sparked a war which lent influence to the more militaristic and extremist leadership within the Mutaren - a zeitgeist that has endured for centuries of conflict.
This extremist view has been sadly sustained and reinforced by interactions with the Juren Empire, who are not shy about their own 'rightful claim' to the whole of the Rutaren galaxy. The ongoing struggle between the two superpowers has only galvanized Mutaren thinking and spurned them onward to further conflict and conquest.
Luckily, the Mutaren have yet to be seen in the Sirian System and, much like the Juren, appear to be content monopolizing the rest of the galaxy for the time being.