I went off to the races with horses, then came back and checked the descr_model_battle.txt file to confirm.

There are two speeds of horses in RTW: fast and normal.

Fast uses fs_fast_horse skeleton and is on "light horse" and "medium horse"

Normal uses fs_horse skeleton for "heavy horse" and "horse cataphract" and generals horse"

I don't have a copy of the file before 1.2 but I suspect that medium was using the fs_horse skeleton before.

It seems wrong that cataphracts are this fast... Seems like speed should be light, medium, heavy/cataphract.