The Stalker was the result of an experiment by C19 in which they injected a Doberman with Stahlman's ooze. (PROSE: The Scales of Injustice)
History
When the Third Doctor exposed C19's secrets, Pale Man managed to take the Stalker with him. Later, he used it as a hunting dog in SenéNet.
When Melanie Bush infiltrated SenéNet HQ, Pale Man was ordered to hunt her with the Stalker. However, the Irish Twins, Cellian and Ciara, killed it when they regained their conscience. (PROSE: Business Unusual)