Headless Horsemen Reins
The Horsemen's Reins drop from The Headless Horsemen who can be summoned in Scarlet Monastery: Graveyard during the Hallow's End event in October. Players can summon the Headless Horseman via the patch of dirt behind the Pumpkin Head. The Headless Horseman fight is a rather easy fight even though when Hallow's End rolls around and you queue up for this instance you will get a lot of fresh max levels with absolutely terrible gear...
With the group finder system players acquire this mount in an entirely different way than before. I am not sure if it has a chance to drop off of the Headless Horseman anymore but I am positive that it comes from the bags that you get for completing the daily for this instance every day. This of course makes the mount more difficult to farm than before, however you shouldn't worry too much about that as it seems like the drop rate was upped just for the bags.
(Taken from Wowhead. Credit goes too Slimda of wowhead)
Headless Horsemen Strategy
The Headless Horseman is to be found inside the Graveyard of Scarlet Monastery during the Hallow's End period, which lasts two weeks. He's a three-phased boss, which is able to be killed with four average players.
Inside the Graveyard, an altar will appear, with an pumpkin upon it. Behind it, is a pile of dirt. By speaking to the pumpkin, you can get a candle, which delivered to the dirt pile behind, will summon the boss. He'll soar out, fly about a bit, and then land, charging the summoner. This will consume a Daily Quest. I've been able to summon him three times in one day, once. This requires you to consume your daily at first, then finish the quest line from the newbie-villages, and then a second daily after that quest line. Summoning him requires level 65.
The Fight, stage one
Tank'n'spank. Nothing exciting happens. You tank, the others do damage, and the healer heals.
As soon as he hits 1%, his head will pop off, and land at a random location. If this head doesn't get damaged down to about 67% before his body hits 100% again, you have to fight through phase one again.
The Fight, stage two
Ok, head reaches ~67%, flies back to his body, and you're on. This time, he has a new ability. Conflagrate. It'll do 10% damage for 5 ticks on a random target. It also does a measly 20 +/- fire-damage to anybody in the vicinity of the target, once per second.
Once he hits 1%, his head pops off again, so try to hit it down to 33%.
The Fight, stage three
The head hits 33%, he becomes whole again, and you're on, for the third time. This time, he gets the ability to summon four helpful pumpkin-adds. If you're quick, they should only have time to spawn before the Headless Horseman is dead. If not, well, try to kill the horseman anyway.
The pumpkins don't do very much damage, but can be very disruptive to the healer. The Headless Horseman will summon four adds every now-and-then, while conflagrating and hitting the tank as usual. At 1% his head flies off, and you've got a floating skull. Kill it quickly, and rejoice. Now go and loot the various drops he has.