Cygnus, the swan, is the name of an astronomical constellation, of which Deneb is the alpha star (See: Deneb).
According to Wendelle Stevens, the cygnusians are an alien race, members of the group under the jurisdiction of the "High Council of Andromeda," but very different from humanoid races. Allegedly they have "large bulging eyes, a very wide spade mouth, no hair, and a dark oily-looking skin, possibly even wet-looking." This "high council of Andromeda" would be linked to the Federation of Planets.