Joss Stilwood

Joss Stillwood is a background character in the second season. He is played by an unidentified extra and appears in "The Wolf and the Lion." Joss Stillwood is the squire of Ser Gregor Clegane.

Season 1
Joss Stillwood squires for Ser Gregor Clegane, the Mountain, in the Tourney of the Hand. After Ser Gregor is defeated in a joust by Ser Loras Tyrell, who was riding a mare in heat to upset Ser Gregor's stallion, he orders Joss to bring him his greatsword. Ser Gregor then beheads his horse in anger.

In the books
In the A Song of Ice and Fire novels, Joss Stillwood is the squire of Ser Gregor Clegane.