Demonic hounds are rare creatures similar to core hounds, but made of green fel energies instead of lava.
The ones in Shadowmoon Valley were spawned from the Hand of Gul'dan, but they are not unique to Outland since Baelmon the Hound-Master summons them from a world on the other side of Death's Door.
In the model files, they are called felbeasts, but in WoW that is a type of fel stalker. No overall name has been given to them. Wildlord Antelarion only calls them "deadly".