A Hound was a title given to a type of shock troopers used by the Halcyonites.
These Humanoids wore masks that match their name and made them frightening to fight. What was even more frightening was the identity of the occupant within; the Hounds were actually Wraith. The origin of these creatures made them a potent force on the battlefield as they were allowed to feed on their Human enemies and even serve as a method of enforcing the peace among the civilian population.
These dominated Wraith were easily cowed on the Halcyon homeworld due to the disruptive influence of the Dolmen which made the Wraith feral creatures that could easily be controlled.
Each of the many kingdoms on Halcyon possessed small cadres of Hounds with the ruling faction maintaining power due to their numbers of Hounds under their command. Most Hounds were created from captured Wraith offworld though the ruling Halcyonite kingdom actually knew of a sleeping Hive Ship on their world and would awaken the hibernating Wraith to serve as their soldiers.
After the Dolmen was destroyed by Lieutenant Colonel John Sheppard with a Puddle Jumper, the Hounds were set free but had been driven insane by the Dolmen and hunger. The Hounds attacked viciously, killing anyone in their way and often not even feeding on their victims. The Hounds were killed while the hive ship was destroyed along with all of the remaining Wraith in stasis. (SGA: "Halcyon")