The Usurper's dogs is the name Viserys Targaryen gave to Lord Tywin Lannister, Ser Jaime Lannister, and Lord Eddard Stark, for the role they played in the rebellion leading to Robert Baratheon's reign on the Iron Throne. Daenerys Targaryen also uses this term to refer to them, blaming them equally for the Sack of King's Landing.
A Dance with Dragons
When Ser Barristan Selmy describes to Queen Daenerys Targaryen how he came to leave King's Landing, he mentions he was present for Lord Eddard Stark's execution. The name inflames Daenerys, and Barristan tries to explain that Eddard was not guilty of the same crimes as Tywin Lannister against House Targaryen. However, she is adamant in her opinion that all the Usurper's dogs are collectively guilty... until this reminds her that she locked up Rhaegal and Viserion for Drogon's attack on Hazzea.
And with him stood the great lords her brother had named the Usurper's dogs, cold-eyed Eddard Stark with his frozen heart, and the golden Lannisters, father and son, so rich, so powerful, so treacherous.—thoughts of Daenerys Targaryen
You do have some dogs you want dead, as I recall.
Barristan: Eddard Stark played a part in your father's fall, but he bore you no ill will. When the eunuch Varys told us that you were with child, Robert wanted you killed, but Lord Stark spoke against it. Rather than countenance the murder of children, he told Robert to find himself another Hand.Stark, what difference? Viserys used to call them the Usurper's dogs. If a child is set upon by a pack of hounds, does it matter which one tears out his throat? All the dogs are just as guilty.