Zanbato (3.5e Equipment)
From D&D Wiki
+2 to attack rolls against mounted targets
The Zanbatō resembles a large katana. This weapon is from the oriental lands originally and was used by infantry against cavalry. In the hands of a skilled warrior, this weapon could kill both the rider and sweep on through to kill the horse below it, too. Such a weapon however required a descent amount of strength and training to use effectively. Due to the size of these weapons they were often very difficult to make, and a smith that could make a quality Zanbato could display it as a sign of their skill.