Species 571 is the Borg designation for a race of the Delta Quadrant who worship a god named Brothara. They were a humanoid species distinguished by a bifurcating (cartilaginous) ridge running vertically down the forehead stopping at the nose.
Members of this species came into contact and were assimilated by the Borg sometime before the 2370s, when a Borg sphere crashed on Planet 1865-Alpha, severing the three surviving Borg drones from the Collective. Among the drones was Lansor, who managed to escape the Collective permanently some time before 2376. (VOY: "Survival Instinct").
When a small group of Borg drones crashed on Planet 1865-Alpha in 2368, one of the drones remembered his former existence as P'Chan, a member of this species, when being separated from the Borg Collective. (VOY: "Survival Instinct")