The Spirits are an alien race native to PXY-887. When the Goa'uld brought the Salish people from Earth millennia ago, the Spirits drove the Goa'uld from the planet. In natural appearance they are humanoid and breathe through gills on their faces, but they are able to adapt their form to change their appearance, and to change the form of others to make them disappear. They adopted the form of the Salish spirits, including Xe'ls the raven and T'akaya the wolf, in order to protect and coexist with the Salish in harmony without interfering in their natural evolution.
Determined to protect their planet from intruders, the Spirits were angered by SG teams attempting to mine trinium on their world. Although the Spirits infiltrated the SGC, causing personnel to vanish and threatening to destroy the base for interfering on their planet, they were convinced by O'Neill to spare the lives on Earth. They returned to their world and revealed their true form to the Salish, whom they continued to protect.
Cross Reference: Captain Conner, PXY-887, Salish, T'akaya, Tonané, Xe'ls
Episode Reference: Spirits