Wraith

From Memory Alpha, the free Star Trek reference.

Not to be confused with the Pah-wraiths, enemies of the Bajoran Prophets.

The Wraith were a shapeshifting species, native to the planet Dakala. They were hunted by the Eska.

While visiting the planet in 2151, Captain Jonathan Archer and the crew of the Enterprise came upon the Eska hunters. One of the Wraiths appeared to Captain Jonathan Archer as a woman he recognized – she had read his mind and taken the form, Archer later realized, of the "glimmering girl" in Yeats's "Song of Wandering Aengus" as he had imagined her when he was a child. She explained that her people are shapeshifters, and the Eskans hunt them using sensors to track them by identifying the Wraiths' chemical signature. Dr. Phlox made a compound that masked their chemical signature concealing them from the hunters and hid them from the Eska's sensors. Once her people had protected themselves with Phlox's compound, Archer's mysterious friend thanked him and let him see her true serpentine form. (ENT: "Rogue Planet")

The unnamed Wraith woman was played by Stephanie Niznik.