Headless Horseman's Mount

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

Riding Requirements:

This mount is available to all eligible characters on your account.
Level 10
Apprentice Riding (ground)
Expert Riding (flying)

Source:

Loot-Filled Pumpkin in Scarlet Monastery (Event Version).
Only available during Hallow's End

Notes:

This unholy steed can only be obtained during Hallow's End, which occurs around mid-late October of each year. Check your in-game calendar for the precise dates of this festival. During the event period you can use the Dungeon Finder to access a special festival boss, the Headless Horseman. The mount may drop from a special loot bag you receive after defeating him, once per character per day.

Introduced in:

Patch 3.0

Travel Mode:

Ground (+60% or +100% speed)
Flying (+150%, +280% or +310% speed)

Speed depends on your riding skill.

More info on Wowhead:

Headless Horseman's Mount taught by The Horseman's Reins