While technically part of the Equestrian Intelligence Service, the Witch Hunters are a largely independent organization, hunting down horrific beasts and unholy magics which could threaten peace within Equestrian borders. While they largely prefer to take in true ponies, they would never turn away the services of one as durable as a Vampire.
When faced against the evil hidden just behind the scenes, some among the Witch Hunters find themselves going mad, turning on their former comrades. Given the damage a deranged Witch Hunter could do, there are those specially trained to put down those who lose themselves. Hunters of Hunters, from those far more powerful than a regular pony, such as Demon-possessed, Half-Demons, and Vampires.