Jamieson, Fausset & Brown

Then Simon Peter, having a sword, drew it, and smote the high priest's servant, and cut off his right ear. The servant's name was Malchus--None of the other Evangelists mention the name either of the ardent disciple or of his victim. John being "known to the high priest" (Joh 18:15), the mention of the servant's name by him is quite natural, and an interesting mark of truth in a small matter. As to the right ear, specified both here and in Luke (Luk 22:50), the man was "likely foremost of those who advanced to seize Jesus, and presented himself in the attitude of a combatant; hence his right side would be exposed to attack. The blow of Peter was evidently aimed vertically at his head" [WEBSTER and WILKINSON].