Remember those horrible cracked-out sketches of D as a bird? Well this butt-ugly chicken thing is responsible for that 8D Reading the Vampire Hunter D novels long enough, you start to realize just how bizarre, fucked up and just plain weird some of the monsters are. So this guy would fit right in. Basically, this chicken-monster was developed by the nobility to fight the nobility. It's a bird with the power to curse, exchanging its stumpy bird form with that of the noble's supernatural grace and beauty (that's the same bird, beauty-fied in a humanoid form in the top there). While this beast, in the guise of a beautiful man or woman roams the frontier eating hapless villagers and seducing humans, the vampire who was cursed into bird form is stuck there, trying to figure a way to get home without getting eaten by a cat or something. The horrible bird retains its beauty, so long as the noble, stuck as a bird is still alive. If the noble should die, the curse is lifted and the beast returns to its true, ugly form.
In turn, if the beast can be captured and killed, the curse will be lifted off the noble and they regain their vampire form.