Xeno was a Ph'Sor.
The Tzun had him and other Ph'Sor communicate with various believers in extraterrestrial life, such as Robert Agar, as part of their invasion plan. They were going to start a nuclear war, then stop it. With the groundwork they had laid, they believed they would be welcomed as saviours by the peoples of Earth. The Ph'Sor claimed to be from Venus, with the one who talked to Agar calling himself Xeno. (PROSE: First Frontier)