The Thracians were a humanoid race native to the planet Macedon III.
At some point prior to the days of early Human space exploration, the Thracians race and culture had largely been devastated in a long and costly war. At around this time, an ethnic group of settlers from Earth arrived on Macedon aboard primitive sleeper ships. Unencumbered by the Prime Directive, the Human colonists intermingled with the Thracians and began to dominate the culture. (TOS - Star Trek II Short Stories short story: "Under Twin Moons")