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 was first encountered by Captain Jean-Luc Picard in 2366, before the Battle of Wolf 359. In 2375, the Queen stated the designation of her species, when she kidnapped Seven of Nine. (Star Trek: First Contact; VOY: "Dark Frontier")
In VOY: "Unimatrix Zero, Part II", the Borg Queen stated, she was assimilated around the same age as a young alien child in the episode, suggesting her species was assimilated well before the 2360s.