Zabrak
this near-human species had several sub-species, each of which was distinguishable by the vestigial horns which ringed their heads. They were known for their ability to withstand great amounts of pain. The Zabrak races were native to the planet Iridonia, but discovered the ability to travel through space early in the history of the Old Republic. They quickly set out to colonize more hospitable worlds after a group of Duros explorers provided them with the knowledge of spaceflight. The strong will and self-confidence of the Zabrak led them to establish a number of colonies on remote worlds, each as loyal to its colony as to their homeworld. One of the largest settlements of Zabrak away from Iridonia was located on the planet Talus. Among the unique aspects of the Zabrak physiology was the presence of a secondary heart. Both muscles worked together to supply blood to an individual's limbs and organs, and had to work in synchronization in order to ensure proper bloodflow. Any arrhythmia in one heart could cause both to start fibrillating, which swiftly led to death if not corrected, because the bloodflow would be essentially stopped as the two hearts worked against each other.