What makes units able to ride on horses?

[Kraxis]

But that was what I did initially... and that didn't work. CTD...

If I understand you correctly that is.
You say I should add the line for the horse skeleton from the general? Or just as a skeleton?

[EDIT]
Found the problem... Every line in the mounted skeletons need to be added. Add one or two and the game will CTD. Now my mounted standard bearer works. Sadly he is not holding his standard upright, and sometimes it passes through the horse's neck. I guess it is better than nothing, but it is not as good as I hoped.

[EDIT2]
Setback!
It doesn't appear that the greek officer and the roman early centurion are compatible with the mounts. They become those 'centaurs' I mentioned earlier. In fact the greek officer is quite funny to watch as he goes into the combat stance with the shield in front of him (right into the neck of the horse). So obviously something needs to be done about this as the standard bearer wasn't up to specs either.