well, AA presented me with the nifedited Mongol Swordsman (and I think he removed some unnecessary parts in blender, like the riders legs and horses head). and I just repainted his texture.
The trick is to move the rider to the front, make it child of the horses and make it look connected and then just use the old animation
well... it looks actually a bit more complicated in nif viewer, probably AA can answer this best.
Here's what I do to make the centaur units, in process order. Only 18 easy steps.
1) Pick the mounted unit who's animation is the closest match to what i want the centaur to do.
2) Import into Blender.
3) Remove all unneeded body parts, human legs, horse head, saddle, etc.
4) Without moving any nodes or entire meshes, just nodes, make the top of the horses neck conform precisely to the shape of the bottom of the riders waist. Make sure that all these nodes are skinned to only the bone you expect (human waist horses neck) or they will break apart in game.
5) Make whatever other modifications to the unit you desire for aesthetics.
6) Export as a .Nif.
7) Open in Nifviewer
8) Add the original mounted unit to this file
9) Copy from the original to the modified, all of the nodes that blender lost. Effects, sounds, and weapon nodes are most common. Also replace any unaltered but misplaced peripheral items such as swords and shields.
10) Also copy one other node to the modified unit and place it as a sibling to the HorseNeck node under the last spine node (I don't remember the precise names right now but will check later) rename it to something useful like "False Neck"
11) Remove the original unit from the project, you are done with it. (Note: if you forgot something, save and close the nifviewer, then reopen it and reload the unaltered file; adding the same file twice in one session causes a crash.)
12) Move the bones from part of the horse that used to be the neck and head onto the "false neck" node and rename them all with a prefix to break the animation link. I add "Frozen" to the beginning of the name for these nodes.
13) Duplicate all of the position and rotation data from the original neck onto the "False Neck" Steps 12 and 13 allow the neck to be frozen for attachment with the torso, but still having the front legs move correctly.
14) Move the waist node (and all children) of the rider onto the "False neck"
15) Modify the position and rotation data of the moved waist node such that it alignes preciscely with the neck of the horse
16) Modify the texture data for any pink objects to map to the correct texture files.
17) Save file
18) Test in game. If the animations look wrong, you can try freezing more nodes by changeing their names. Often freezing the human spine nodes one at a time has good results.
I have never made it through all of these steps in one shot, usually i get aways into nifviewer before noticing something i need to change in blender, then i get to restart from step 5.
"
Probably a good thing I no longer make fantasy units. 
