From A Wiki of Ice and Fire
The Usurper’s dogs is the name Viserys Targaryen gave primarily to Lord Tywin Lannister and Lord Eddard Stark who were Robert Baratheon’s main supporters during the rebellion. Daenerys Targaryen uses this term to refer to them also.
|“||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. ||”|