He is mentioned within the entry of Ser Gwayne Gaunt, as recorded in the Book of Brothers. During the Defiance of Duskendale, he killed Ser Gwayne. For this, he was later killed by Ser Barristan Selmy.
In the books
In the A Song of Ice and Fire novels, Symon Hollard was a key accomplice in the Defiance of Duskendale, when Lord Denys Darklyn of Duskendale defied King Aerys II and captured him, imprisoning him within the Dun Fort.
Ser Symon slew Ser Gwayne Gaunt of the Kingsguard during the taking of the King. When Ser Barristan Selmy of the Kingsguard entered the Dun Fort and escaped with the King, he avenged Ser Gwayne by killing Ser Symon.