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. It was the first flying horse mount in the game, and the only one to do so without wings.

The saddle is buried in the cemetery of the Scarlet Monastery.

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.

Mount Journal
Be it into the flood, or into the fire, this one will go where you require.

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