Defeating the Horseman for this evil little gourd is no easy feat. It requires a full group of L70s, one of which needs a Daily Quest given out only during the Hallow's End event. Once the Horseman is defeated in the Scarlet Monastery graveyard instance, the new pet then only has a 25% or so chance of dropping.
For you pet completist out there, this is a must have. For everyone else, it definitely tops the cool list. Hallow's End is scheduled to begin October 18 and run daily through November 1.