Species 125 was the Borg designation for a humanoid race which, by the 2360s, had been assimilated into the Collective.
One notable member of Species 125 was the Borg Queen, who claimed to belong to this species when she kidnapped Seven of Nine. (VOY: "Dark Frontier")
In VOY: "Unimatrix Zero, Part II", the Borg Queen stated that she was assimilated around the same age as a young alien child in the episode, suggesting her species was assimilated well before the 2360s.