Once a very highly advance civilization. Their planet exploded because of an instability in the core of the planet around 100 yrs ago. Many of them fled the planet and try to colonize other worlds but their isn't much info about that for lack of communication.
A Borg drone was rescued by the Wolfheart who originated from that civilization, even if he had been taken when he was 10 yrs old, he revealed certain physical characteristic of his people. Some of his people have middle empathic power, and some could even transformed in animals, or even various humanoid species. This process was done on the molecular level but the individual can only maintain the shape for one to two hours at the most and this after a lot of practice. Physically, they have one characteristic, their eyebrows are small dots.
The female of the species are as tall as the males, the only exception are the size. The males are a lot more bulky then the females. The life span is like of a human. Their biological immune system is more advance then ours. The normal gestation period is 12 months.
So far, we haven't found any of their colonies but it's just a question of time. From what we know, they come from a sector not yet explored by our probes.