The Thracians are a group of Indo-European tribes inhabiting a large area in Southeastern Europe.
Long before the events of Spartacus: Blood and Sand, Thrace was a country ruled by "Spartacus the Thracian King", otherwise called Sparatocos, son of Teres I. He ruled the Odrysian Kingdom from 450-431 BCE, and was a warrior that fought like an animal, taking on multiple enemies with a wild combat style that prevailed in legend. This name was passed on through tale across generations, reaching the tongues of Romans such as Senator Albinius, who would later name the Thracian slave Spartacus, in honor of the combat style they shared as well as their race.
At some point in the past, the southern Thracian tribes became embroiled with the Getae, a powerful and hostile neighboring tribal confederacy that adorned themselves in animal skins and bones, wearing white makeup to cover their bodies. They had a reputation of burning villages, raping women and stealing everything the Thracians possessed. As such, Spartacus and those of his village desired their end.
Striking a deal with Claudius Glaber, the Thracians of the Maedi agreed to aid in his campaign against Mithridates, as long as the Romans agreed to kill the Getae - all of them. Thrace had a reputation amongst the Romans (especially Claudius Glaber) of being a dirty country run by animals. Therefore, it was not often that the Romans and Thracians are allied militarily. The Romans did not honor their deal and instead ordered the Thracians to march to attack the army of Mithridates, ignoring the Getae threat to Thracian villages. This sparked a rebellion, in turn killing several Romans and incapacitating Glaber. This distraction allowed the Getae to advance and destroy many of the Thracian villages, likely destroying many Thracian tribes, while many of the survivors (including Spartacus) were enslaved and brought to Capua. Here, all but one died in the Arena, and that lone survivor would be named after the legendary Thracian King, becoming Spartacus.
The Thracian people were generally long limbed with dark to red hair. Many famous Thracian heroes are known to have had Red hair and blue eyes. It was thought by the Greeks that red hair originated from Thrace.
In The Show
In the show, the Thracians are portrayed as being fierce/adept warriors but highly undisciplined, violent, unpredictable and uncontrollable which makes them difficult, yet rare, slaves in the Roman Republic. Despite their barbaric appearance, the Thracians are more intelligent and cunning then they appeared to be.