AuðumblaBos chthonii

Auðumbla or just Umbla (Bos chthonii, "chthonian cow" in reference to their planet's namesake) are the most populous species on Tartarus and make up the majority of the lower class. All Tartareans are ruled by the negligent king, Helhest, and the royal family of Sleipnir. They evolved from Humi cattle.

Umbla are generally earnest, diligent, and peaceful, yet the hardships faced by living under Tartarean conditions has pushed many of them to extremes to survive. They do not have a positive outlook of the nobles nor royalty, although many remain faithful to the imperial family, seeing their inaction as both "redeemable" and a "grand design meant to endure" (they are, in fact, actively brainwashed by the Bursti). Not a day goes by without some sort of revolt, and many revolutionary groups function from the shadows, intent on demolishing the monarchy. More than anything else, Umbla are tenacious, hardy, and steadfast in their goals; once resolved to do something, they will achieve it. (It would be wise, perhaps, for the blue bloods to stop poking a sleeping bear...)

Many Umblans—particular those living in the outskirts—are hunted by the nobility for sport, and thus live life either in hiding or on the run; the nobles will sometimes force inner Umblans to the outskirts to "refill" their hunting grounds. Umblans are also often forced to participate in noble-led, gladitorial arenas upon capture (if they didn't already willingly enter them for the chance at supplies). 

By the upper class (sans the royal family, who are by and large reclusive), they are demeaned for their "inferior" social status; insults such as commoner, peasant, etc. are frequently flung their way by the Gullinbursti especially. ("Cow, pig," etc. are usually insults in and of themselves.) 

Umbla take on the various bovine characteristics. They are herbivorous, thus making their diets even more difficult to come by within Tartarean domes.