Ah, a Dark Eldar question...
The Kashnarak can be controlled by a Wych unit only. When I wrote the rules it was supposed to be a Wych formation, but the problem arose when a Wych formation was reduced to 2 units (neither of which were Wyches... say that five times fast). It was then changed to a Wych unit.
The whole idea was that these deciated gladiator types would in a sense have the particular know-how on herding these beasts without getting eaten. Hellions and Reavers would be a bit caught up in their flying and hovering to do that IMO.
Thanks for the interest and be sure to put Dark Eldar in your subject line next time. I frequently check the boards for questions relating to the dark kin.
