Simon Toyne is a character referenced in Game of Thrones who died a long time before the series began. He is not expected to appear in the series.
Simon Toyne was the leader of the Kingswood Brotherhood, a band of outlaws and bandits opposed to the rule of House Targaryen. They inhabited the Kingswood, a large area of woodland south-east of King's Landing. Roughly twenty years before the events of the series, King Aerys II Targaryen ordered the Brotherhood destroyed, a mission led by Ser Barristan Selmy of the Kingsguard. Selmy himself killed Toyne with a counter-riposte, a move witnessed by the then-squire Jaime Lannister and later cited as one of the finest sword moves he'd ever seen.
In the books
In the Song of Ice and Fire novels, Simon was a descendant of House Toyne. House Toyne was formerly a house of honor, with one member serving in the Kingsguard of King Aegon IV Targaryen. When Aegon IV found this knight abed with one of his mistresses, he had him executed in a horrible fashion. The knight's brothers declared war against House Targaryen, slaying the infamous Dragonknight, Ser Aemon Targaryen. Simon was their descendant and the last of his line.