What I would do is just have a cutscene or something showing D'Artagnan's father giving him the horse (but not get on it). Then, you would have to have a whole 'nuther unit to rename, since you can't rename created units. After the cutscene is over, show him going down the road already on his horse, and name that unit D'Artagnan. Of course all this is assuming you were trying to use the "horse+man=rider trick" (or something like that). You cannot rename created units in the AOK Editor. If you can't understand this, then post a reply and I will try and give you more detail.
4ID OIF 07-09
The engineer said before he died
There were two more drinks that he'd like to try
The conductor said What could they be?
A hot cup of coffee and a cold glass of tea