An attack dog is a specially trained East-European Shepherd specializing in anti-infantry attacks appearing in Red Alert 1.
In an effort to counter Allied guerilla formations and to prevent the infiltration of their infrastructure and command, the Soviet Union trained large numbers of German and East-European Shepherds, Huskies, and other species of dogs to sniff out and kill enemy infantry, particularly Spies.
As they are relatively inexpensive, the Soviet Union trained a frighteningly large number of attack dogs during the war and housed them in specially-built Kennels on the battlefield.
Attack dogs were also used in the Third World War, although kennels were no longer needed; the dogs were trained in the barracks. The attack dogs were mostly used to guard Soviet bases from infiltration by Allied spies. However, the Allies saw the potential of attack dogs and began to use them too, although in much smaller numbers than the Soviets did.
While understandably quite useless against tanks, they were a fearsome threat to Allied infantry; the canines can kill any infantry in one bite. They are not fooled by disguises, and are especially useful against unarmed enemy spies and engineers.
Of the thousands, possibly millions, of canine warriors in the ranks of the Red Army, one stood out - the legendary attack dog Chitzkoi, who accompanied the Soviet supersoldier Volkov. Chitzkoi was armored and extremely resistant to enemy fire, almost as strong as Volkov himself, and was able to lunge huge distances at enemy infantry. He was still helpless against vehicles, but his ridiculously heavy armor allowed him to survive all but the heaviest and sustained anti-armor attacks.