The Miradorn and the Acamarian Gatherers are common interstellar raiders, and may make good minor antagonists for your campaign. They can be found on space stations, as pirates, or as mercenaries under the employ of others.
The Miradorn, from the planet Mirada, are a species of telepathically-linked twins. Miradorn ships, also called Theta-class raiders, are popular ships among mercenaries. They appeared in the episode “Vortex” on Deep Space 9.
Acamarian Gatherers had chosen to leave their homeworld instead of seeking peace with the factions on-planet. They adopted the lifestyle of nomadic raiders, and sometimes cooperate with other species. They appeared in the episode “The Vengeance Factor” on Star Trek: The Next Generation.
Both species are presented as Minor NPCs, along with stats for their starships.