The Husnock are an extinct species destroyed in 2366 by a Douwd who had assumed the Human identity of Kevin Uxbridge. Uxbridge described the Husnock as "a species of hideous intelligence who knew only aggression and destruction." Their physical appearance is unknown.
In 2366, a Husnock warship attacked and destroyed a Federation colony at Delta Rana IV. Unknown to the Husnock, an energy lifeform known as a Douwd was also living on the planet in the guise of Uxbridge. In a split-second of anguish over the Husnock attackers killing his wife, the normally pacifist Douwd destroyed every member of the Husnock race with its enormous powers. The Husnock numbered fifty billion at the time. (TNG: "The Survivors")