Horsemanship is a mechanic that exists on only 36 cards, all of which came from the Portal Three Kingdoms set.
I originally went over Horsemanship in my review of From the Vault: Legends, and he’s still an amazing card now due to his ability to give all of your creatures Horsemanship. Like Flying, Horsemanship can only be blocked by creatures that also have Horsemanship. Due to the fact this ability only exists on 36 cards in all of Magic, this would make your entire army unblockable.
You can find more information on Horsemanship here.