Harry, Ron and Hermione see the knight with an arrow sticking out of his forehead speaking to the Fat Friar at Sir Nicholas de Mimsy-Porpington’s 500th Deathday party on Oct. 31, 1992 (CS8). Before his death, he was a wizard (OP38).
It is possible that this is the ghost of could be Harold II (1022? - 1066). He was the last Saxon king of England. He was defeated at the Battle of Hastings in 1066 by William the Conqueror. Harold was killed when an arrow pierced his eye. Thanks to Mike Ball again for this excellent deduction.