The Horseman's Reins

The Horseman's Reins teaches you to summon the Headless Horseman's Mount which serves both as a flying mount and a ground mount.

Source
This rare mount can be looted from Loot-Filled Pumpkin which is awarded once per day to players who use the Dungeon Finder to form a group to kill the Headless Horseman during the Hallow's End seasonal event.

Achievement
Obtaining these reins will reward the Feat of Strength The Horseman's Reins (achievement).