The Borg Queen, former member of Species 125
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 a Borg Queen, one of which was first encountered by Captain Jean-Luc Picard in 2366 before the Battle of Wolf 359. In 2375, the second Queen encountered 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 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.
In the Star Trek: Legacy segment "The Origin of the Borg", it is stated that the Collective found that the females of certain species (likely referring to Species 125) displayed a mental prowess that enabled them to sift through thousands of thoughts and bring order to chaos.