Use the collision box from basic creature mesh if the X one have no collisions
Currently guars from the Imperial Guar Transport mod (not sure if an english version is available) have no collisions, because xGuar_withharness.nif file has no bounding box.
This mesh works fine in the Morrowind (probably because the Guar_withharness.nif has collision box).
So I suggest to take a bounding box from basic file, if actor's X*.nif has no bounding box.
Edited by Andrei Kortunov