The Corsair class is something of an enigma. First shown in the class demonstration from Fire Emblem: Rekka no Ken, it appears to be equivalent in every way to the Pirate class. The only noticeable difference is in the class descriptions, which allude to Corsairs being good at fighting in lakes whereas Pirates are good at fighting in seas, despite pirates having equal movement to the corsairs in lakes. Also, corsairs are unable to promote.
Matters are further complicated by Corsairs being rarely found in the game itself, being confined to Chapter 23A: Four Fanged Offense (Linus's version). There are no named units in this class, only generic Black Fang members.