N'Kai

The N'Kai are an alien species that have existed before even the planet Earth. Currently, they are fighting a losing war against the Space-Spawn. Their war has lasted for many centuries, and they have positioned Sentinels on various worlds to watch for Space-Spawn invasions of those planets. The only known member of this species is Nokkar, who appeared in the episode Sentinel.

N'Kai are basically "humanoid" in appearance, and they are extremely long-lived. They seem to have no trouble breathing the Earth's atmosphere.