Species 125

From Memory Alpha, the free Star Trek reference.

Species 125 is the Borg designation for a species as yet unknown to the Federation outside of the Collective. However, it is likely that this species has been completely assimilated.

The Borg Queen, as of 2375, was a member of species 125 before being assimilated, and thereby had the body of a female of this species prior to having a cybernetic body constructed for her. (VOY: "Dark Frontier")

There are faint horizontal lines on the Borg Queen's neck. This may either be a species characteristic or a Borg neck implant.