Ok,regardless of their single attack,30 point cost,vulnerability to magic missiles or 5-10 unit size...Should you ever buy a couple of Black Knights/Hexwraiths boxes,consider this simple conversion work:
-cut the heads of the skeletal horses
-cut the riders off at the torso
-cut off the rider`s heads (optional)
-attach the torso to the horse`s neck joint to form a centaur like thingie
-attach the skeletal horse heads to the rider`s neck...
(+1 to whomever "gets it") I know I will make a few of these.