Horsemanship is an evasion ability associated almost exclusively with Portal Three Kingdoms. A creature with horsemanship can generally be blocked only by another creature with horsemanship, making it extremely difficult for most Magic decks to stop in combat.
Sun Quan, Lord of Wu can give an entire creature army horsemanship, while Lu Bu, Master-at-Arms combines the ability with haste. Because horsemanship appears on relatively few cards, it often behaves like an especially potent form of evasion.
