Cygnus, the swan, is the name of an astronomical constellation in the Northern hemisphere, of which Deneb is the alpha star (See: Deneb). It features a prominent asterism (pattern of stars), known as the Northern Cross.
Cygnus - screenshot from Stellarium
1. 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.
Cygnus as depicted in the Urania's Mirror cards
2. Several sources also refer to one or more civilizations from the Deneb system. (See Deneb for more information).